photo: QST ad 1-38, Stan Wolff of the
NY Herald Tribune copying Press with the 'Super-Pro' - this is an
interesting ad in that it shows both the SP-10X (right) and the SP-100X
(left) models.

Was the Hammarlund Super-Pro the ultimate pre-WWII communications receiver? The
advertising of 1937 stated it was "The Last Word" in communications receivers.
Other advertising called it "Tomorrow's Receiver - Today." Its build
quality was second-to-none. As Hammarlund said about designing the Super-Pro,
"...at no time was cost considered a limiting factor." Hammarlund wanted
to build a "standard" by which other communications receivers were measured. The
Super-Pro was advertised as an "amateur-professional receiver" and it had several unique features that did set it apart from its
contemporaries. These included a virtually sealed precision tuning assembly
with custom designed variable condensers and a cam-operated knife-switch type 360 degree rotation bandswitch,
variable-coupled air-tuned IF transformers that allowed a continuously
adjustable IF bandwidth and a powerhouse, high fidelity audio output. In
the hands of an experienced operator, the Super-Pro could out perform
any other receiver. Listing at $400 and selling at discount dealers for
about $250, not many Depression-era hams could afford the Super-Pro, so
not much was ever written about the receiver in ham magazines. As a
result, among hams, opinions are highly diverse when it comes to the
Super-Pro's performance capabilities. The best pre-war receiver? Let's
see,... H.Rogers, Feb 2008

The Incredible Pre-war 'Super-Pro'
Receivers

Hammarlund's Company History up
to 1931

Oscar Hammarlund came to the United States
from Sweden in 1882 to
work for the Elgin Watch Company. By 1886, he had gone to work for
Western Electric in Chicago. He later
worked for the Gray National Company on the Teleautograph machine and
finally, in 1910, formed his own company, Hammarlund Manufacturing
Co., Inc. At first the company built gadgets but they soon became
involved building Western Union call boxes. An interest in wireless led
to the Hammarlund designed variable condenser which became an industry
standard for quality. In the mid-twenties, Hammarlund formed a temporary partnership
called Hammarlund-Roberts specifically to supply Hammarlund-built parts
for broadcast radio kits. As home radio technology improved, the
popularity of radio kits declined and Hammarlund-Roberts
Co. folded. That certainly didn't affect the operation of Hammarlund
Manufacturing Co., Inc., in fact, they were just coming out with their newest
creation, a shortwave radio that was going to push the few
communications receiver manufacturers of the time into the superheterodyne building business.

The
Comet and the Comet-Pro - 1931 to 1935

Hammarlund's entry into the shortwave and ham receiver
market began in 1931 with the introduction of the Comet All Wave
Receiver. The early Comet circuit used eight tubes, plug-in coils
(without shields) and operated with a
built-in AC
power supply. However, what was important was that it was a superheterodyne.
In fact, the Comet is generally credited as the first
successful commercially-built shortwave superhet, introduced at a time
when almost all shortwave receivers in use were TRF with regenerative
detectors. The Pilot AC Super Wasp was typical of type of short wave
receivers in use at the time and the Comet was superior in all
comparisons. Of course, the Pilot Super Wasp had been a kit and was
considerably cheaper than the Hammarlund Comet. Also, most of the hams
at the time built their own receivers that were usually simple and
inexpensive. The Hammarlund Comet was beyond what most hams at the time
could "homebrew."

By late-1932, the Comet had evolved into the
Comet-Pro and this version of the receiver changed the way in which commercial users, the military and
the amateurs listened to shortwave signals. Hammarlund described the
Comet Pro in their advertising as "The World's Finest Shortwave
Receiver." It really wasn't an exaggeration, the Comet Pro was a
superbly built, great performing receiver - at least when compared to
the other receivers of the early thirties. Up to the introduction of
the Comet-Pro, the Regenerative Detector with a couple of TRF stages
ahead of it was thought to be the most sensitive receiver on shortwaves.
The superheterodyne, while fine for AM Broadcasts on medium wave, was
too noisy and that lowered its usable sensitivity - at least that was
what most shortwave radio enthusiasts thought. Hammarlund took the
challenge to build a shortwave superhet that had low internal noise and
was sensitive. By careful design work using quality parts and
construction, the Comet-Pro did achieve what was thought impossible - a
usable shortwave superheterodyne receiver. As the Comet evolved to the Comet-Pro, it
was fine-tuned into an excellent performing communications receiver that
became quite popular, ultimately being found in many ham shacks of the
thirties and also
used by many professionals, including the military. Some Comet Pro
receivers even made it to the Arctic and Antarctic with various
expeditions.

By 1934, the Comet Pro was beginning to show its age.
Its
limitations were due to the lack of an RF amplifier, making the
Comet-Pro susceptible to images as the received frequency was increased.
Some users would add an after market pre-selector to eliminate the image
problem. Other issues included lack of a calibrated dial readout, a BFO
control that was a "swing-arm" lever accessed under the lid and the fact
that changing bands required changing a set of two plug-in coils. The
competition was beginning to displace the Comet Pro from its number one
status. National had come out with their AGS and FB-7
receivers and was busy designing their fabulous HRO. Patterson had the
PR-10 with a matching preselector available and Radio Manufacturing Engineers had their RME-9 receiver with
a TRF amplifier built-in, direct frequency readout on the tuning dial
and an "R" meter. No doubt, Hammarlund wanted to return to the days when
they offered the only "high performance" receiver available. The
Hammarlund engineers had been at work on the Comet Pro's successor since
1933..... More information on the Comet Pro receiver at the
bottom of this webpage in Appendix A.

photo left: Comet Pro ad from Radio News
February 1933

The 'Super-Pro' Production History

photo above: Right page of the QST ad
from March 1936

Designing the 'Super-Pro' began in 1933 when Hammarlund announced it was working on a new shortwave receiver dubbed
the "Comet Super-Pro." At that time there were only a few superheterodyne shortwave communications receivers on the market -
National had their AGS and FB-7 receivers and Patterson may have just introduced their "All
Wave" predecessor to their PR-10. However by 1935, several manufacturers
such as Breting, Patterson, RME and RCA had
all come out with fine quality superhets, not to mention that National
had released their fabulous HRO receiver. As the competition continued
to build better and better receivers, Hammarlund, still in the design
stages, continued to improve the concept of what the 'Super-Pro' would
be. That Hammarlund wanted to produce the best communications receiver
available cannot be denied. A receiver that would appeal to engineers,
professionals and well-to-do hams. Quality of construction would be of
the highest caliber and performance would be to the limits of the design
possibilities of the day. By March 1935, Hammarlund dropped a hint that
the Comet Super-Pro would be released soon but the year
went to conclusion with no formal announcement. By June 1935, Hammarlund
was supplying a Super Pro receiver, the SPA, to the Signal Corps. Finally, in the March
1936 issue of QST, a full two page ad announced the "Super-Pro"
receiver. A personal letter from Lloyd Hammarlund accompanied the
detailed introduction advertisement. By the thirties, Lloyd Hammarlund
was generally running things at the company although his father, Oscar
Hammarlund was still company president (Oscar H. died in 1945.) The
introduction told about the great Super-Pro features and showed what the
receiver looked like, inside and out. Touted as an "Amateur-Professional
Receiver" no where in any advertising is the price ever mentioned.
Listing at $400, the Super-Pro was one of the most expensive of the generally available
communications receivers in the mid to late thirties.

photo above: Left page of the QST ad
from March 1936

The Super-Pro Features
- With the availability of the new Super-Pro receiver hams
and commercial users now had a communications receiver that boasted
several innovative features. The precision that was designed and built
into the RF tuning unit was incredible. 25 coils mounted on 20 Isolantite
(a type of ceramic material with low loss characteristics) bases working
with a Hammarlund-built 4 gang tuning condenser and 12 gang bandspread
condenser along with a custom designed cam-operated knife-switch type bandswitch made up the heart of the RF
tuning unit. The main tuning dial featured band-in-use masking. The
antenna input coils have a Faraday shield between the primary and
secondary to keep the input impedance constant regardless of tuning. The bandspread condenser has three
different sections per gang that optimizes the variable capacitance
needed for 90% span on the 80, 40 and 20 meter ham bands. The RF box is
an entirely shielded, nearly sealed metal box and only connects to the IF/AF chassis with eight
wires (and the four grid leads.) The receiver tuned from .54 up to 20MC in five bands. Bandspread
was provided for the upper three tuning ranges. Another innovation was
the variable-coupled air-tuned 465KC IF transformers that were part of the
continuously variable Selectivity control. With this control, IF bandwidth and audio
fidelity were adjustable from 3 KC out to 16 KC. Of course to take
advantage of this, a full fidelity, high power audio amplifier was
provided with a transformer coupled, triode-connected, push-pull audio
output delivering about 14 watts of power. An Amplified AVC system was
used for maximum control of the RF/IF gain when listening to SW-BC
stations or local AM stations. A "Tuning Meter" measured the total IF
amplifier plate current and, as strong carriers increased the AVC bias,
the IF gain was reduced and the meter would read less current. The
operator tuned for the lowest meter reading while tuning in an AM
signal. There was a procedure for logging the "no signal" noise level
current and then measuring the tuned signal current and then calculating
the "db over the noise" as a signal report - Hammarlund thought all hams
and engineers carried a slide rule in their pocket! Also included was a front panel controlled
Beat Oscillator (BFO,) a Tone control along with
separate RF, IF and AF gain controls. 14 tubes were used in the
receiver. The separate power supply used two tubes, a 5Z3 B+ rectifier
and an 1-V rectifier for the bias supply.

photo left: Super-Pro ad from QST July 1936 - CE William
Greer, WNEL, where the Super-Pro was used to rebroadcast WABV

Super-Pro Model Designations
- When the Super-Pro was first introduced it was just
referred to as "The Super-Pro" as there was no need to distinguish it
from any other models. This situation changed quickly as different
frequency ranges, crystal filter options and speaker size options became
available. For example, an "X" suffix designated a crystal filter
option while an "S" suffix referenced the ham version of the
receiver. By the time the 100 Series was introduced, several options
were available. They were designated as follows:

When the receiver left the factory it had a speaker
included with it. Two choices were available with the 100 Series, a 10"
or a 12" speaker. These speakers were supplied without an
enclosure. To designate which size speaker was included the
number indicator was modified as follows:

The other suffix designators for the other receiver
versions follow this same pattern. Note that the two example receivers
would be identical but with different size speakers supplied.
>>>

>>> Nowadays, Super Pro receivers are rarely (if
ever) found with their original speakers so designating the original
speaker size becomes superfluous. It might be a bit of curious
information that the receiver originally came with a 10" speaker but
since that speaker is no longer with the receiver, it isn't important.
It's likely that the lack of any original Hammarlund speaker enclosure
on the early Super Pro receivers led many hams to build their own or
incorporate the speaker into a wall baffle. This undoubtedly has
resulted in the original speaker being "left behind" while the receiver
(and, hopefully, the power supply) were sold or otherwise "relocated."
This article references all Super-Pro models to the "Series" that they
belong to as this eliminates the confusion of the two speaker size
options changing the model designation of identical receivers. Thus an
SP-110X is identified in this article as an SP-100X, belonging to the
"Super-Pro 100 Series" which is how Hammarlund designated the receivers
in their advertising.

Manuals
- If you are in need of a manual or are looking for the schematics in
Rider's Perpetual Troubleshooting Manuals, you will find that the
speaker size designator is used to identify the receiver. Thus, an
SP-100X is listed as an SP-110X or SP-120X. Again, it makes no sense
today to reference the speaker size that was included with the receiver
but that is how many Super-Pro receivers are listed in various manual
suppliers and in Rider's manuals. When ordering a manual, be sure to
make allowances for the model and speaker designators versus how the Super-Pro
receivers are referenced in this article.

SP-200 Series
- The SP-200 Series receivers follow the same designations as the SP-100
Series.

The Super-Pro Models
- 1936 to 1945

The "Super-Pro" -
SP-10
1936

Officially introduced
in the March 1936 issue of QST magazine*, initially the SP-10 was just
called the "Super-Pro" - there was no need to distinguish it
specifically since there were no other models. The SP-10 designation
comes into use around the time the "100 Series" was introduced. The first
production Super-Pro used all glass tubes with vented tube shields on
all tubes except the audio section tubes. The
tube lineup was RF - (2)6D6, Mixer - 6A7, LO - 6C6, IF - (3)6D6, DET - 6B7,
AVC - 6B7, BFO - 6C6, 1st AF Amp - 76, AF Driver, P-P AF Output - (3)42.
The power supply used a 5Z3 for B+ and a 1-V for the bias.

The push-pull amplifier was capable of 14 watts of high
fidelity audio power. The AF output transformer was specified in some
sources as a 600 ohm Z line audio (Rider's, for instance.) However, the
SP-10 manuals specify that the output Z is 8 ohms and the schematic
contained in the manual marks the output as "Speaker Voice Coil."
But, there is also another published SP-10 schematic that shows an added
phone jack output along with the phones and speaker terminals with
resistors added in series in the audio output line. Unfortunately, there
are no values shown for the components on the schematic. One could
assume since the audio output line is marked "600 Ohm Line" that the
resistors comprise somewhat of a match when used with the standard 8 ohm
Z output transformer secondary. The speculation is that the audio output
transformer was always an 8 ohm Z secondary and no real change ever took
place. It's the same transformer with the resistors added for 600 Ohm Z
and the resistors not installed for 8 ohm Z, (so far, all SP-10
receivers encountered have an audio output Z of 8 ohms.) Connections were on the rear apron of the
chassis and were heavy-duty fiber-mount pin jack sockets for earphones
and the phono input and screw terminals for speaker. The phono input
allowed the user to access the high fidelity, high power audio
amplifier. Additionally, there are separate RF, IF and AF gain controls
along with a Tone control. None of the controls had any sort of
calibration or reference scales.

* The SPA
receiver was the SP-10 produced for the Signal Corps in June 1935 - see
more details in "The Military Super-Pro" section below

photo
above: The
original Super-Pro from 1936, later designated as the SP-10

The Tuning Meter was more than just a signal strength indicator.
Since it measured total IF amplifier plate current, it could also
indicate when the IF amplifiers were being overloaded by too much RF
signal. It was able to cope with a variety of RF and IF levels and still
maintain a useable indication. Additionally, it also would function on
CW which meters that functioned off of the AVC line couldn't do. The
meter wasn't illuminated. The tuning dial featured a rotating dial mask that was gear driven by
the bandswitch and provided a "band-in-use" tuning scales. Three
variable coupled 465KC IF transformers were used in the adjustable
Selectivity control. The spring-loaded plunger from each IF transformer
is cam adjusted by levers mounted under the chassis. The Selectivity
shaft is also spring loaded to provide a "positive feel" to the control.
In addition to the front panel adjustable IF Selectivity, the Input and
Output IF transformers to the detector and the Output transformer for
the amplified AVC had adjustable coupling. The adjusters were knurled
nuts on threaded shafts the protruded out the tops of the IF cans. The
user could set the detector bandwidth for optimum selectivity for the
receiver's particular service. The use of Input and Output IF
transformers along with a 6B7 duplex diode-pentode tube for a second
detector-IF amplifier resulted in the SP-10 actually having four IF
amplifier stages and ten tuned IF circuits.

Within a few months the
"S" version was announced in QST. This receiver tuned from 1.2 to 40MC,
so hams had available to them a Super-Pro the would tune in all of the
amateur bands from 160M to 10M. Additionally, the bandspread was changed
to allow its use on all tuning ranges. This version was advertised
extensively in the ham magazines during the last half of 1936.
There was also a Crystal Filter now available. The Crystal Filter
mounted in the upper left part of the panel with the 465KC crystal
assembly and the phasing condenser mounted to a small panel with an
on-off switch and a scale for the phasing condenser. The front panel had
to have a cut out to allow the Crystal Filter panel and assembly to be
mounted. >>>

>>> It is interesting that none of the advertising artwork
ever shows the early Super-Pro with the Crystal Filter option installed. The power supply
provided terminals for connection of the speaker field coil. The SP-10
power supply provides slightly lower B+ voltages than the later supplies
but the difference is less than ten percent. An electrodynamic speaker
was included in the purchase price but it was just a speaker - no
housing was provided. The standard speaker was 8" in diameter but for an
extra $25 a deluxe 12" speaker could be purchased.

It is apparent that engineering changes were incorporated into the SP-10
receivers through out its short nine month production. Perhaps the
earliest change was to the biasing of the 2nd Detector Input amplifier
grid (6B7.) This change added a resistor to the bias series resistor string by changing
the 600 ohm resistor to two 300 ohm resistors which allowed the bias
voltage to be slightly increased by moving the connection to the junction
of the two 300 resistors. This arrangement is also found in the later SP-100
receivers. The BFO circuit was also changed to have a parallel grid
capacitance rather than capacitance to ground connection. Additionally,
the BFO plate load resistor was changed from a 5K resistor to a 50K
resistor, though this may have been a parts list error that was
corrected in the SP-100 series. The most visible change was the
elimination of the Tone control, which was incorporated towards the end
of the SP-10 production. Hammarlund probably believed that
very few of the Super-Pro owners were using the Phono input for
phonograph record reproduction and therefore the Tone control was
unnecessary since for radio reproduction the Selectivity control would
do about the same thing - limit the higher audio frequencies. The SP-10
series was in production for about nine months and the total number
produced was probably around 500 receivers. Though the list price was
$400, most discount dealers sold the SP-10 for around $250.

The SP-10 and Front-End Overloading - Hammarlund stated in the SP-10 manual that it was possible to
overload the front-end of the receiver by tuning in very strong, local signals and
advancing the RF gain too much while in AVC. The manual even gives an
example of what to expect if the RF Gain was advanced further than
necessary when receiving local signals. The overloading was especially
likely if an efficient antenna system was used on the two lowest
frequency bands.
The fact that the RF Gain can be advanced sufficiently when receiving
strong signals to cause overloading and the fact that Hammarlund does
cover this in the manual seems to suggest that it was a deliberate part
of the SP-10 design. Hammarlund's engineers
did
re-vamp the RF-IF Gain control circuit and replace the adjustable
Detector and AVC transformers for the new SP-100 Series, making it
seem likely that they were aware of the ease at which users could
misadjust the SP-10. The engineering changes to the SP-100 eliminated
the overloading while in AVC issue. Today, there is no evidence that
overloading was a common "problem" experienced by SP-10 users when it
was new. Over a period of time, component degradation, modifications,
changes in wiring layout and mechanical assembly due to repairs will
certainly affect how well an
SP-10 functions today. By loosening the screws on the bottom cover of
the RF Tuning Unit, one can easily induce RF overload and distortion
with just a slight advance of the RF Gain control. Also, poor solder
joints in the RF AVC line will accomplish the same thing due to improper
AVC control of the RF amplifiers. There is no evidence
that the SP-10 had overloading problems when new and to accomplish
overloading
required substantial misadjustment or modification of the receiver.
>>>

>>> As an example, the SP-10 shown in the photo below once was
in use at WMI.
It was modified and repaired over the years and after its tenure at WMI
it was further modified and repaired in a "creative" manner that changed
the much original wiring layout and component placement. This SP-10
did easily overload on strong signals in AVC, so much so that the RF gain
bias source was changed by a former owner to help solve the problem.
However, after a complete "rebuild to original" of the receiver that included
returning to the
original wiring layout and the original component placement
along with complete (correct) mechanical assembly, this SP-10 now
performs flawlessly, exactly as the manual describes it should operate.
That is, when tuned to a strong local AM BC station (10KW 15 miles
away,) using an efficient BC band antenna (outside 50ft end fed wire,)
with the RF Gain at maximum in AVC, some distortion is apparent.
However, by reducing the RF Gain to about 50% the overloading is
eliminated and so is the distortion (just as the manual states.) When
the SP-10 is connected to my 135' tuned dipole, I can tune in the same
AM BC station and have the RF Gain at maximum with no distortion at all.
This is due to the inefficiency of the tuned dipole at AM BC
frequencies reducing the level of the input signal, thus the RF Gain can
be substantially higher. This is exactly how the manual describes the
operation of the SP-10. This should establish that the SP-10 was
designed to be a highly adjustable receiver, able to cope with many
different types and levels of signals. That it might be misadjusted was
assumed by Hammarlund engineers and those conditions were described in
detail in the SP-10 manual.

Shown above is a 1936 SP-10 sn: 576. This Super-Pro was used at
WMI, a ship-to-shore station located in Lorain, Ohio on the shores of
Lake Erie. This receiver has been completely rebuilt and restored to
original "as delivered to WMI" in 1936. Its terrific
performance has tremendous audio range and incredible sensitivity. The variable
selectivity is totally rebuilt and functions great.

Photo above: WMI operating room in 1937. Note the three
SP-10 'Super-Pro' receivers in the racks. SN:576 is certainly one of the
receivers in the racks. Photo from: Inland Marine Radio History
Archive - here is their URL:
http://www.imradioha.org/WMI.htm

The 100 Series "Super-Pro"
1937-1939

photo
above: Introduction advertising for
the new SP-100 Series - QST, January 1937

The new "100 Series" Super-Pro was introduced in January 1937. It
upgraded the receiver's front-end by changing the large glass tubes
there to metal octal tubes. The two RF amplifiers were now 6K7 types.
The mixer was a 6L7 and the local oscillator was a 6J7. Also, the audio
section was changed over to metal octals with a 6C5 1st AF amplifier
with a triode connected 6F6 driver and push-push, triode connected 6F6s
for the output. In all, eight tubes were changed over to metal octals
while the IF amplifiers remained 6D6s, the 2nd Detector and AVC
amplifier remained 6B7s and the BFO remained a 6C6. The power supply
B+ rectifier remained a 5Z3 and the bias rectifier was changed to a type 80.

Other changes were
elimination of the separate RF and IF gain controls. The RF and IF
stages' bias lines were combined to use a single manual control labeled
"Sensitivity." The Sensitivity control could also adjust the AVC bias line
more negative than the AVC control bias and therefore reduce the overall
gain while still maintaining AVC control above that point. The variable
coupled Detector and AVC transformers were replaced with fixed-coupling
units. Since there were now no moving parts in these transformers, the
component boards that had been mounted under the chassis of the SP-10
were relocated inside the Detector Output transformer can and the
Amplified AVC Output transformer can. The elimination of the separate RF and IF gain controls and the
variable-coupled transformers for the Detector and AVC were probably due
to the ease at which the SP-10 could be misadjusted, resulting in
overloading and distortion. The Selectivity control, as it was called on
the SP-10, was renamed as a Band Width control. The Tone control was
also dropped because the user could limit the audio highs by reducing
the IF bandwidth. Actually, it is likely that the Tone control was
eliminated on the later production of the SP-10.

The BFO and Bandwidth controls were given front panel calibrated
scales. Even the Sensitivity control and the Audio Gain were given
reference scales. Internal changes also included different AF
transformers that were vertical mount, frame-type (non-potted) units
with the AF Output transformer having a single 8 ohm output Z. These
were new style audio transformers and not the same style potted 8 ohm Z
transformers that were installed in the late SP-10 production. Since a
switch was provided for "Speaker" or "Phones" the earphone Z is rather
low. A 600 ohm line audio was not provided in the SP-100 series except
for perhaps the very late versions of the SP-100LX version. The standard speaker size was changed to 10"
with the introduction of the SP-100. The AVC also
could be used for CW as a large time constant capacitor (.25uf) is
switched into the AVC line when the receiver is switched to CW. Of
course, the receiver could also be used in "Manual" control for maximum
sensitivity in the CW mode.

The SP-100 series still used the "Tuning Meter" that measured total
IF amplifier plate current resulting in a "backwards" operation of the
meter. The 0 to 5 scale on the meter actually has ten divisions for each
numeral - 50 divisions total. The push-pull audio remained at the same
power capabilities of 14 watts and a Phono input was still provided as a
rear connection. A "Remote Relay" operation of the B+ was added to the
rear chassis. The input is via pin terminals and the function parallels
the operation of the "Send-Receive" switch. When the Crystal
Filter option was installed in the SP-100, the first IF transformer was
changed from a variable-coupled unit to a fixed coupled transformer with
leads brought out the front for connection to the crystal holder
mounting and the phasing condenser. Whether this was the case with the
earlier SP-10 is not known as most examples of that receiver don't have
the Crystal Filter installed.

The Super-Pro's antenna input Z is around 115 ohms. This was
approximately the impedance of the twisted pair antenna feed lines that
were popular for dipole antennas. Since the Super-Pro doesn't have an
antenna trimmer control, Hammarlund expected the user to provide a
matched antenna. Most professional users and most knowledgeable hams
provided matched antennas but shortwave listeners sometimes used end fed
wires or inverted "L" antennas with no matching device. The normally
high impedance of these antennas didn't match the Super-Pro input Z and
the common result was higher noise level along with weaker signal
strength.

The cabinet for the early Super-Pro and the SP-100 is really nothing
more than a dust cover that is secured with eight knurled thumb nuts on the
front and three thumb screws on the rear. There is a metal
identification tag mounted on the back of the dust cover with places for
the receiver type and serial number to be stamped however most of the
time these are stamped with some kind of factory
identification - usually two letters. Serial numbers usually do match on
the dust cover and the chassis. The bottom cover has small
metal-cup, felt-center feet mounted in each corner. This gave the user a
table top cabinet. The panel is 18" across on the table top
models. A 19" rack-mount model was available and, with the Crystal
Filter option included, the list price was nearly $450. The front panel
on early, pre-WWII Super-Pro receivers is made of .187" aluminum.
This was black wrinkle finished first, then engraved. This resulted in
the nomenclature looking bright silver.

The model designators used
with the SP-100 Series were as follows: SP-100 covered .54 to 20mc and
had no crystal filter, SP-100L covered 100kc to 400kc and 2.5 to 20mc,
SP-100S covered 1.25 to 40mc. If the crystal filter was added to any
version then an "X" suffix was added. Hammarlund's ordering
department needed to specify which size speaker was to be included with
each receiver package ordered.

Photo above: Article-ad from POPULAR MECHANICS
12-37 promoting the Super-Pro SP-100SX for 10 meter coverage. This
article shows that the speaker included was just that - a speaker - no
cabinet was included. Note also that this article never refers to the
receiver by its model number - it is only a "Short-Wave Super De Luxe."

To this end, when ordering a Super Pro, "SP-110" indicated that
the 10" speaker was to be included and "SP-120" indicated that the 12"
was to be included. Even though the speaker size that was included as
part of the entire receiver package was referenced by the numeral
modification, that model number is never stamped into
the metal tag on the back of the receiver dust cover. Hammarlund
advertising always referred to the receiver as the "Super-Pro 100
Series" or the "SP-100 Series."

Rider's Perpetual
Troubleshooter's Manual mentions the SP-100LX version and indicates that it
substituted the 2.5 to 5.0mc band with a 150 to 300kc band. This is in direct conflict with the Hammarlund SP-110LX manual (that is
available on the BAMA site.) That manual indicates the coverage is
100-200KC and 200-400KC and 2.5-20MC in the remaining three bands. This
is the standard "L" coverage, even for the later SP-200 Series. However,
even some of the SP-200 advertising also mentions the 150KC-300KC tuning
range. There may have been some special orders with that frequency
coverage but standard production provided 100kc to 400kc.

Another interesting "L" feature,
that is not found on most "LX" receivers but is shown in the BAMA site
manual, is dual secondary windings of the output transformer and a front
panel "Phone" jack for the phones. This became the standard for the
later SP-200 models but perhaps is only found on the late version
SP-100LX models or maybe "special order" receivers. From actual
examination of SP-100LX SN:2730, this receiver has no front panel phone
jack but has the standard Phones-Speaker switch with separate outputs on
the rear chassis. Phones out is for pin jack connection while the
Speaker out is screw terminals. The nominal output impedance appears to
be around 8 Z ohms for both Phones and Speaker, which is standard for
the SP-100X receivers. (See photo of SP-100LX serial number 2730 in
the Collector's Gallery further down this page.)

Until more
SP-100LX information becomes available one can only speculate on what
was "standard" for the SP-100LX receivers. This is what appears to be
standard. The SP-100LX tunes from 100kc to 400kc in the first two bands
and from 2.5mc to 20mc in the highest three bands. Audio output
impedance is 8.0 Z ohms for both Phones and Speaker. A toggle switch
selects either Phones or Speaker for the audio output. There is no Phone
jack on the front panel.

Photo above: 1937 SP-100X Super-Pro (SN: 3387) - This
receiver originally belonged to Pacific Gas & Electric Company in
California. Since it was owned and maintained by PG & E, it was never
modified and was only repaired as necessary. See the section further
down this page about our authentic restoration of this great example of
the SP-100X.

There is another interesting SP-100 variation that has
been reported by SMØAOM. This receiver is known as the MRM-5, or Markradiokottagare 5, and it was sold only to government customers in
Sweden. The receiver is an SP-100 but the frequency coverage is 200kc to
400kc and .54mc to 10mc. The receivers were available in either rack
mount or table top configurations. SMØAOM has two examples, sn 1352 and
a partial sn 16xx. The Swedish translation manual is dated October 1937.

The SP-100 series was in production for about two years and nine
months. Though the list price was over $400, most dealers sold the SP-110X (ten inch speaker included) for
around $250. Total number of SP-100 Super-Pros produced was probably
around 1000 receivers.

The SP-150 1938

Beginning in July 1938, Hammarlund offered a customized SP-100 receiver in a floor
standing console cabinet. The cabinet included doors that closed over
the receiver panel and a 15" speaker with a bass reflex port. The receiver panel was faux
finished walnut and had gold tone nomenclature along with brown knobs.
The meter housing and dial escutcheons were trimmed gold and the dial
masks were amber color instead of black. To assure that the receiver
would appeal more to the shortwave listeners and home radio users,
Hammarlund eliminated the front-panel control of the BFO, making it an
internally set adjustment. The Crystal Filter option was not installed
either. Otherwise, behind the panel the SP-150 receiver was a standard
SP-100 series set.

What must have sounded like a great idea to Hammarlund,
considering the success of Scott and McMurdo Silver high performance
receivers, turned out to be a sales flop. The SP-150 was a superb receiver that was really never appreciated by very many people in
the radio buying public. It just wouldn't do for hams - it had no BFO on the
front and it was too big. It didn't have the Scott or McMurdo chrome
chassis - how could
you impress your friends? The rather plain cabinet couldn't compare to
the flamboyant creations from Scott or McMurdo. As a high priced console,
the SP-150 couldn't find many admirers even though it performed as well as any Scott or McMurdo and had a more accurate dial
readout. Only about 70
were ever built, making it one of the most difficult Super-Pro receivers
to find.

Shown in the color photos is the SP-150 owned by
AA6S, Bill Jungswirth. The cabinet has been refinished and the
replacement grille cloth is not the original style or color. The
receiver has been totally rebuilt and performs very well. The close-up
of the front panel shows the faux walnut paint job of the front panel
along with the gold nomenclature and the amber dial masks. A spectacular
console that performs as well as any Scott or McMurdo.

The 200 Series "Super-Pro"
1939-1945

Introduced in October 1939, the "200 Series" thoroughly modernized the
Super-Pro to 1940 receiver design concepts along with a reduction
in the build-cost of the receiver. The SP-200 is a more conventional
approach to superheterodyne design, especially in the IF section of the
receiver. The SP-200 used three IF amplifiers with standard 465KC IF
transformers (except for the two variable coupled units for Band Width
control) and the 6B7 duplex diode-pentode detector-IF amplifier of the
proceeding Super-Pro receivers was
replaced with a standard duplex diode 6H6 tube. The AVC circuit was also
simplified using capacitive coupling rather than an inductive pickup and
reducing the transformers to one unit. All of these changes were a
cost-to-performance decision resulting in a receiver than performed as
well as its predecessors but was much less expensive to build. The list
price reflected these changes by a reduction to $315.

The circuit now used
all octal tubes replacing the glass tubes of its predecessor with metal tubes. Tube lineup was RF-(2)6K7, MIX-6L7, LO-6J7,
1IF-6K7, IF-(2)6SK7, DET-6H6, NL-6N7, AVC AMP-6SK7, AVC REC-6H6,
BFO-6SJ7, 1AF-6C5, 1AF&P-PAF-(3)6F6, B+ REC-5Z3 and Bias REC-80 ...
18 tubes in all. Additionally, the "200 Series" did away with the IF
current "Tuning Meter." Now the S-meter was illuminated
and had a 0 to 9 scale. The meter operated from the AVC line so it
worked normally instead of "backwards." Undoubtedly, the old tuning
meter's complicated method using the "delta" of signal versus noise
calculation to give the "db over the noise level" had almost everyone
confused. Complaints must have been common and ultimately prompted the
change to a standard S-meter. A new Noise Limiter was incorporated into
the circuit. It was a "clipper" type noise limiter. The Crystal Filter
assembly was now entirely installed inside the first IF transformer.
Also, instead of only a Phasing adjustment, a five-step Selectivity
control was added. Now, for the first time, the Super-Pro was installed
inside a real metal cabinet with hinged lid and carrying handles and the
front panels were standard 19" rack width on all models . The
cabinet will have a metal ID tag mounted on the back and sometimes a
model type will be stamped, or perhaps a serial number, but many times
these tags were never stamped at all. If the model type is stamped in,
it usually is "200" and a letter suffix to further identify the
receiver, e.g., "200X."

The "200 Series" evolved slowly. The first evidence of change is
in the SP-100 front panels no longer using the large square cut-out for
the Crystal Filter panel. Instead, the Crystal Filter assembly is
mounted directly to the front panel with just holes for the controls. Next, were the four
toggle switches for AVC/Manual, Speaker/Phones, Send/Receive and Mod/CW.
Some very early "200 Series" may have all four switches, but quickly the Send/Receive and Mod/CW
switches are changed to rotary switches with knobs and the position of
the Send/Receive switch is changed. Finally, the Speaker/Phones switch
is replaced with an earphone jack. With the earphone jack change, the
audio output transformer also changed to a dual secondary outputs and
sometime later both audio transformers became potted units (as the
original SP-10 was.) The dual audio outputs provided 600 ohm Z line
audio to drive a matched speaker and a separate Hi-Z winding to drive
earphones or other Hi-Z device. The dual secondary windings and the
front panel "Phone" jack features were also found on the SP-100L
versions but, for some reason, not on the first SP-200 receivers. Though the layout of the receiver is
very similar to the SP-100, the underside of the chassis uses different
component mountings and the shield between the RF and IF sections was
removed.

The three versions of the SP-200 are the SP-200X, tuning from .54 to
20.0MC. The SP-200SX, tuning from 1.2 to 40MC. The SP-200LX, tuning from
100 to 400KC in two bands and 2.5 to 20.0MC in three bands. Like the
SP-100 series, the
Hammarlund sales literature uses SP-210 if the version ordered came with
the standard 10" speaker and SP-220 if the version came with the deluxe
12" speaker. Special "universal" power supplies could be ordered for
operation of 115vac or 230vac. Sales literature also sometimes indicates
that the LX version covered 150 to 300KC substituted for the 2.5 to
5.0MC band. Interestingly, Rider's also lists this coverage for the
SP-100L version. Most likely these are both errors. The earliest known
LX (at present,) SN:8423, tunes 100-400KC and 2.5-20.0MC or the standard
coverage for the LX. An error in the 1942 sales literature
indicates that all versions had bandspread on all five tuning ranges.
Actually, this only applied to the SX version, the X and LX bandspread is only on
the three highest tuning ranges. Earlier advertising had phrased the
statement as "bandspread throughout the entire high frequency range of
the receiver" which is technically accurate.

photo above: The Series 200 Super-Pro as pictured in the
1941 Hammarlund Catalog. This artwork shows the Speaker-Phones switch
found on early versions.

photo above: An artwork image of the SP-200 chassis from
the 1941 Hammarlund Catalog showing the early version frame type audio
transformers. These were later replaced with potted transformers.

The photo to the left-top shows the 1939 Series 200 Super-Pro SP-200SX sn:
6230, covering 1.2 to 40MC. This is the early version with frame-type
audio transformers and using the toggle switch for selecting either
"phones" or "speaker." The front panel is .187" thick aluminum that is
black wrinkle finished and then engraved to have the nomenclature appear
bright silver. Note on the S-meter - though this meter appears to be
similar to the S-meter shown in the artwork images of the early SP-200,
there is considerable doubt that this is an original meter. It has
obviously been rebuilt in the past using parts from two different
meters. It is more likely that the full glass front style was standard
for production until the later military contracts. Interestingly, the
metal tag on the back of this receiver cabinet is stamped "200SX" as the
model number.

Just before WWII started, the aluminum front panel was changed to a
.125" thick steel front panel. When the front panel became a steel fabrication, the
paint used changed to a smooth-finish, semi-gloss black. Additionally,
the nomenclature was stamped into the panel and then filled with white
paint. Most of the steel panels are copper-nickel plated on the front
side only - to prevent corrosion and subsequent problems with the paint.
All Super-Pro panels, from SP-10 to SP-200, are painted on the front
side only. The back of the panel is left unpainted.

The photo left-lower shows the later version of the 200 Series
(SP-200X sn: 9419) with the smooth finish, semi-gloss black steel panel.
Also, the standard S-meter housing with full glass front is shown on this receiver. Note that the toggle switch for "Speaker-Phones" is replaced with a
single .25" phone jack indicating that the dual output audio transformer
is installed. Since
the new steel panels were .125" thick steel, spacers were installed on
the various mountings to compensate for the different thickness of the
panel. It seems likely that all of the civilian
Series 200 Super-Pro receivers probably had black panels. The tag on the
back of this receiver's cabinet was left un-stamped which is common with
many Super-Pros.

During WWII, government
agencies like the FBI used the 200 Series Super-Pro at their
listening posts and most likely these receivers were the civilian
models. When the demand increased during WWII, some variations in the
front panel paint color appear, especially those receivers that were
destined for the Signal Corps. Grays, Green-Gray and Blue-Gray are the
most commonly seen colors. Civilian Super-Pro production is difficult to
estimate because found examples indicate that sales of these versions
continued though the early part of WWII - at least to 1943. SN:16135 is
a civilian X model built in mid-1943. It is likely that only a very
small percentage of the wartime production were destined for
non-military use and then it is likely that certain government agencies
were the only users. SP-200 non-military production is probably around
1500 to 2500 receivers. Confusing this production number is the fact
that the military did purchase communications receivers early in WWII
for use in the war effort. These receivers are sometimes re-tagged and
will have Signal Corps stamps. Hammarlund serial numbers will identify
when the particular SP-200 was built and if the tags or stamps post-date
the serial number then the receiver was probably "drafted" into military
service.

It is very common to find some variations in the SP-200 Series receivers but
whether this is due to production stock variations, special orders or subsequent repairs
using later stock parts can't be verified accurately without a physical
inspection of the receiver to determine the originality of the parts or
assembly in question.

The Military "Super-Pro"
1935-1945

The Super-Pro was selling to the military on a small scale before
WWII. In June 1935, the U.S. Army Signal Corps ordered the new Super Pro
receiver under contract (order) number 10932-NY-35 with the receiver designated as the
SPA. Although this contract is nine months ahead of the official QST
announcement in March 1936, it seems likely that the SPA was delivered
long
before the civilian versions were available. The SP-100LX was also
produced for the Signal Corps and may have had a military designation
but known examples only seem to have Signal Corps acceptance stamps and
a few other military stampings. With the SP-200, the early military versions
(pre-WWII) are
acceptance stamped but have no special designators known. The volume of Super Pros
going to the military increased dramatically with the start of WWII.
Early WWII versions are similar to the pre-war SP-200 receivers in that
no special designators were used. Acceptance stamps are usually the only
indication of military service.

By 1943, the military versions were being identified by
their US Army Signal Corps designations. The "X" was known as the
BC-1004, while the "LX" was the BC-779. The least seen version is the "SX"
which was known as the BC-794. There are other military designations
that usually pertain to specific changes made to these standard models -
usually frequency coverage differed from the standard models, e.g.,
R-129/U covered 300kc to 10mc. However,
sometimes the end user also would require special designations. In
particular, Allied users may have required special designations. There
are usually suffixes that designate which power supply came with the
receiver. The WWII Super-Pros were sometimes built by contractor Howard
Radio Company, though the only non-Hammarlund Super-Pro versions seen
are the BC-779 and power supply. Under the chassis the changes to the circuit mostly involve component
types. Seventeen of the individual paper-wax capacitors are replaced with metal
container units - the so-called "bathtub" capacitors that usually
contain multiple capacitors inside the housing. It is likely that the
1941 and later civilian models also had the "bathtub" caps installed.
These "bathtubs" are mounted on the inside wall of the chassis in the RF/IF
area. Additionally, there are a few tubular oil-filled capacitors used. The chassis may be MFP'd depending on its service. >>>

photo above: The 1935 Hammarlund SPA Receiver built
for the Signal Corps U.S. Army on contract number 10932-NY-35 dated June
29, 1935, nine months before the official announcement in March
1936 QST magazine. Although this image is artwork and not a photograph, it's
obvious that the SPA is the SP-10 with a military data plate mounted on
the front panel. Artwork is from "Instruction Book for Super-Pro -
Manufactured by Hammarlund Mfg. Co,.Inc., New York, NY, USA - published
by authority of The Chief Signal Officer - Order No.
10932-NY-35 Date June 29, 1935."

photo above: ASP-1004 Receiver - 1945
version of the BC-1004

>>> The standard "BC" receiver was a rack mount unit
with a dust cover that was secured by eight thumb nuts on the front and
three thumb screws on the rear. Dust covers are painted black wrinkle
finish. Though the majority of "BC" receivers are rack mounted units,
sometimes the receiver will be installed in the military CH-104-A
cabinet. This cabinet is similar to the SP-200 civilian cabinet except
there are no decorative chrome strips installed. When the receiver is
installed in the CH-104-A cabinet, the dust cover holes in the front
panel are usually filled with screws and nuts. CH-104-A are found in
either black or gray wrinkle finish. Shown to the right is a BC-1004
installed in a CH-104 cabinet.

There weren't very many changes required by the military, so
WWII Super-Pro receivers are basically the same as the civilian
versions, under the chassis. Front panels are different on the WWII
Super-Pro, especially late in the war. Many of the panels were painted
various shades of gray. Some of the later 200 Series receivers will have
a greenish-gray paint on the front panel of the receiver and power
supply. There are numerous variations in the color, with some models
tending to be more blue-gray. Certainly the highest production
quantities for all of the pre-war and wartime Super-Pro receivers are
the military versions with total production of several thousand units of
each type. Total production was probably around 7500 to 9000 receivers.
Signal Corps TM-11-866 covers the most commonly seen versions of the
military Super-Pro.

Shown in the photo left is one of the later WWII Super Pro receivers with
the greenish-gray front panel paint. This one is designated as
"ASP-1004." Many of the WWII Super-Pro receivers were used by our Allies. Does the prefix "ASP" = Allied Super Pro? Interestingly, under the tag, the panel is ink stamped
"BC-1004" but there are no Signal Corps stamps anywhere on the receiver.

photo left: The Manila, Philippines radio
communications link to Tokyo. Photo is dated August 20, 1945. Note the
BC-221 Frequency Meter. This unit provided an accurate way to set the
Super Pro to the proper frequency. Judging by where the dial mask is,
the "TOKYO" receiver is set on the 10-20MC band. Power
supplies are on top of the receivers and four different receivers are
visible in the shot. photo from C. Cusick, KØDWC collection

photo right: The intercept station at the Yakutat Air Base in
Alaska during WWII. Three SP-200 receivers are in use and one SP-100
receiver (with the black panel.) Judging by where the dial masks are,
the two left side receivers are set to the same band while the right
side receivers are set to different bands. Note the unusual rack
mounted speaker panels at the top of the racks. photo from Fred Bryan, www.oftencold.com

Allied-Built Versions
of the Super Pro SP-200

Some countries built their own military versions of the Super Pro. In
particular, the Australian company, Eclipse Radio, built the AMR-200
which is nearly a perfect copy of the SP-200.

The Russians
produced the KV-M which is built like a Super Pro but without the bandspread dial.
The chassis layout and especially the RF box are very close
copies of the SP-200 Super Pro. Shown in the photo to the right
is a KV-M receiver. This one has an addition of a small
lamp that is mounted to the front panel.

See references at the bottom of this webpage for links
showing photos of the AMR-200 and other photos of the Russian
KV-M receivers. Thanks to Steve
Bringhurst for the AMR-200 and KV-M information.

The R-270/FRR receiver started out as a standard WWII
BC-794 receiver. It's likely that some of the military Super Pros were
never shipped or, if they were, were never put into service by the end
of the war. After WWII, the Signal Corps was interested in developing RTTY communications to the point where it was virtually
"error-free."
Diversity reception assured that the incoming signal didn't fade thus
providing the RTTY TU with a fairly constant level input signal (dual
diversity can reduce fading by up to 90%.) The R-270/FRR is one of the
component receivers of the AN/FRR-12 Dual Diversity Receiver built by
Wickes Engineering & Construction utilizing customized-rebuilt
Hammarlund BC-794 receivers. While dual diversity practically assured
that the received signal remained constant, it didn't help with receiver
frequency drift due to LO and BFO instability (which the Super Pro was famous for.) Wickes incorporated a
selectable Crystal Oscillator that could be switched in to replace the
LO. Of course, the receiver had to be
also manually tuned to the
correct operating frequency so the RF and Mixer stages would be
"in-tune." The operator could also tune in the Crystal
Oscillator harmonics with the Main Tuning which would then have the
correct RF and Mixer tuning for harmonic operation. To prevent drift from the BFO it's operating frequency was
also crystal controlled. The output from the two R-270/FRR receivers was
taken from the IF output (provided by Wickes' modifications.) This was
routed to
a Diversity RTTY Demodulator, the CV-31A/TRA-7 that accomplished the
diversity combining via its internal circuitry. Once set-up, the
AN/FFR-12 could provide virtually "error-free" RTTY.

The R-270/FRR was rebuilt and customized in 1948 by Wickes
Engineering & Construction Company for the U.S. Army Signal Corps. The
contract date on the Signal Corps data plate on the front panel tag is 1947
(and the manufacturer is shown as "WickesENGINEERING & CONSTRUCTION CO.") but the MFP
coating is date-stamped "OCT 1948." The custom modifications consist of
adding a three channel crystal oscillator that can be selected from the
front panel via a switch that provides Crystal Oscillator OFF/ON function plus the channel
selection. Also, on the small Crystal Oscillator panel, which is located
above the main tuning dial escutcheon, is a vernier frequency control.
The Crystal Oscillator modification is the same as Signal Corps
"Improvement Kit" MC-531 which is shown in TM11-866. MC-531 is the
forerunner to the JX option in the SP-600 receiver and functions in
a similar manner. If the Crystal Oscillator is ON, its plate voltage is
derived from the B+ that is switched from the LO plate, thus disabling
the LO. When the Crystal Oscillator is OFF, then the B+ is routed to the
LO plate. >>>

>>> You must have a crystal installed
that is at +465kc from your desired receive frequency and then you also
have to manually tune the receiver to the desired receive frequency, otherwise
the RF stages and
the Mixer will not be in tune (which is exactly how the SP-600JX works.)
The actual crystal frequency used will depend on the desired receive
frequency (band used) and whether or not the crystal is going to be
operated on the
fundamental or on an overtone.

Additional Wickes' custom modifications were to
replace the AVC rectifier tube (6H6) with a 6SN7 dual triode. This
provided the AVC rectification and also provided a triode buffer output for the
IF OUTPUT. Also, the AVC Amplifier tube was changed from a grid cap 6K7
to a single-ended 6SK7 and the BFO tube was changed from a grid cap 6J7
tube to a single-end tube, the 6SJ7. Additionally, the second and third
IF amplifier tubes where changed from grid cap 6K7 tubes to single-ended
6SK7 tubes.

Further changes included a new front panel made out of
aluminum with raised lettering (rather than the Hammarlund engraved or
stamped lettering.) Also, the standard steel bottom panel was replaced
with an aluminum bottom cover that had the receiver schematic applied to
the inside. IF transformer cans were
replaced if the associated tube was changed to a single-ended tube.

The AN/FRR-12 manual (TM11-896) indicates that the
receivers could also be used
for diversity CW or AM. If CW was going to be used then the BFO crystal
(462.45kc) had to be removed and the BFO "re-aligned" to function as
a standard BFO. The user also had to "short out" the crystal socket with
a jumper wire. Like the SP-600, the R-270 was modified by Wickes to use a
"twin-ax" connector for the antenna input. Additionally, a CT was
provided on the audio output transformer (labeled as "600-0-600" - although this
nomenclature actually indicates that the 600 Z ohm output is across the
entire winding, not each side of CT.) The entire chassis and shields were giving an extra
heavy coating of MFP, which is typical of later, post-WWII Signal
Corps equipment.

The power supply provided was the R-74A "Heavy Duty"
military style. This was the later version with the 5U4 and 5Y3
rectifiers. The interconnecting power cable was also heavy duty and
rubber jacket insulated. The power supply is also heavily MFP'd and date
stamped DEC. 1948. The power supply bottom cover has the schematic on
the inside of the cover.

The Wickes' R-270/FRR is certainly
more than just a "modified" Super Pro receiver. I would call it
"customized and rebuilt" for the Signal Corps requirements for drift-free and
fading-proof RTTY reception. The quality of workmanship, both mechanical
and electronic, is superior and what would be expected from a
professional engineering company. If one didn't know how the standard
Hammarlund WWII Super Pro receivers were built, the Wickes' "Custom
Super Pro" could easily be mistaken for Hammarlund factory workmanship.
Performance is identical to the Hammarlund receivers with the exception
that several optional outputs and frequency control circuits are
available. Audio is still excellent, sensitivity quite good and the
variable IF coupling provides excellent control of the bandwidth.

Super-Pro Power Supplies and
the Power Cable

The power supply for the Super-Pro provides the several
different voltages required to operate the receiver. There are three
levels of B+, +385vdc, +270vdc and +140vdc. The tube heater supply is
6.3vac and the C bias is -50vdc. Power is connected to the receiver via
a four foot cable that has a ten spade lug connector that screws to the
ten pin terminal strips on both the power supply and the rear of the
receiver chassis. Usually the protective covers that mount over the
terminal strips are missing. The SP-10 power supply used a type 1-V
rectifier tube for the bias supply. This was replaced in the SP-100
supply with a type 80 rectifier tube. Early power supplies will have a second
terminal strip with two connections marked "Field" for the electrodynamic speaker used with the SP-10 and SP-100 receivers. With
some late SP-100s and all of the SP-200s, a choke was included in the power supply to replace the
field coil of the electrodynamic speaker and allow the use of a PM
speaker. The standard power supply had
a metal box cover with louvers that protected the tubes. The power
supplies were usually placed on the floor near the
receiver. Rack mount power supplies have a 19" panel screw mounted to
the front of the chassis and a metal cover over the top of the chassis
with lugs the protrude out the front panel for the cap nuts. Bottom
covers were standard on all types of power supplies. Metal cup, felt
center feet are usually mounted at the corners of the bottom cover for
non-rack units.

SP-100 PS SN:3388 - Note the two terminals for the field coil of
the electrodynamic speaker

photo above: SP-200 PS Rack Mount - Note that the
terminals for the speaker field coil have been eliminated

photo above: ASP-84B PS Military Rack Mount

Most of the military Super-Pro power supplies will have
an oversize potted power transformer and potted chokes with oil-filled
filter capacitors. Some of the late-version military supplies went to
can-type oil-filled paper capacitors. About this time the rectifiers were
changed to a 5U4 and a 5Y3. All of the military supplies were normally
expected to operate in continuous duty service. Some of the military
power supplies will have dual primary power transformers for 115vac or
230vac operation. Military power supplies for the Signal Corps are
usually identified as RA-74, RA-84 or RA-94. RA-84 is for 115vac
operation only. The RA-74 is a heavy duty, mulitple primary power supply and the RA-94 is
the heavy duty, dual primary power supply.

Fortunately, almost any of the Super-Pro power
supplies can be used with almost any Super-Pro receiver.
Considerations would have to be made for the field coil
connection on early supplies but the voltage output terminals
and the voltages present are the nearly the same for all power
supplies and receivers. The exception is the SP-10 which
requires a
lower B+ because of the type 42 output tubes however the voltage
difference is less than 10% and operation of the SP-10 on a
later supply can be easily accomplished (for testing purposes) by moving the AC lead to the 125vac tap
under the power supply chassis.
This will lower the output voltage slightly and allow safe
operation for testing of the SP-10 with a later power supply. If
long term operation is required using a later PS with an SP-10
receiver, then a 220 ohm 5 watt resistor should be added inside
the power supply between the +385vdc B+ line and the output
terminal. This way the PS primary tap can be on the proper AC
voltage which allows the heater voltage at the receiver to be
correct and also the +385vdc will be reduced to about +365vdc at
the 42s in the SP-10 receiver. The advantage
of using a later power supply with either the SP-10 or SP-100 is
the elimination of the electrodynamic speaker. These earlier
supplies utilized the field coil of the speaker as a filter
choke but later power supplies already have a choke installed so
a permanent magnet speaker can be used.

Unfortunately, since just about any Super-Pro power
supply can be used with just about any Super-Pro receiver, very few
matched set-ups are found today - that is the sequentially serialized
receiver/power supply. With the SP-10 and SP-100 receivers, the original
power supply serial number is one off from the receiver serial
number. The assignment of a sequentially serialized power supply to a
specific receiver ended by the time the SP-200 was introduced in 1939.

The Super-Pro power cables normally encountered are
usually a nine wire, cloth covered cable with a special, spade-lug type
terminal strip connector on each end. These cables were used on the
later SP-200s and the military receivers. Some military power cables are
shielded and have rubber jacket insulation. Later nine wire SP-200
cables have tube heater wires that are of a larger gauge wire than the
remaining seven wires. The tenth lug is not used on these terminal
strips. On early cables, there are usually nine wires that are all the same
gauge. If a tenth wire is present, it is "switched to ground (chassis)
connection" that operates with the ON-OFF switch. It is not used in the
circuit as supplied but was provided as a spare function for possible
user applications. The switched ground operation is on most Super-Pro
receivers but the an actual wire in the cable is seldom present unless
added by a former user. Many Super-Pros will be missing their original power
cable but the cables are easy to make. Just observe that the tube heater
connections, pins 1 and 2, use about 12 to14 gauge wires while the
remaining wires can be 16 to 18 gauge. The cable length is approximately
five feet. The connections to the
power supply match the receiver, 1 to 1, 2 to 2, etc. It isn't necessary
to have the terminal strip connectors but they do make connecting and
disconnecting the receiver and power supply much easier. The correct
orientation for an original power cable is to have the spade lugs
pointing down and the cable to exit the protective cover on the right
side.

photo right: The power cable from a military SP-200
Series Super-Pro receiver-power supply showing the unique spade-lug type
connector strip

Hammarlund Super-Pro Serial
Numbers and Estimated Production

From examining many pre-WWII Super-Pro receivers along
with several HQ-120X receivers and Comet-Pro receivers, it appears that Hammarlund issued serial
numbers in a new sequence starting with "1" (or some low number) beginning
with the first SP-10 receiver and continuing in sequence right through the military versions built during
WWII. Since
the earlier Comet Pro receivers all have much higher serial numbers and were
all built before any Super-Pro receivers, they must have been issued
numbers from a
different, earlier sequence. For example, a 1933 Comet Pro receiver with
the SN: 7822 and a 1936 SP-10 with SN:576. It is likely that all
Hammarlund products that were going to be issued a serial number were
issued numbers from a common serial number roster. Therefore all
Super-Pro receivers were serialized sequentially as they came off the
line. It also appears that when the numbers were issued to a Super-Pro
"set," that receiver's power supply was issued the next sequence number
after the receiver's serial number. This is the case in the two early
matched sets that I know of. One is an SP-10, sn 720 and PS sn 721. The
second receiver, an SP-100X, sn 3387 and PS sn 3388. If this
was the normal Hammarlund practice, then each Super-Pro "set" accounted
for will use two numbers from the sequence - one for the receiver and
one for the power supply. There seems to be some SP-10 and SP-100 sets
that are "matched units" but are not sequential serial numbers. The
numbers are very close but not sequential. It was probably standard
procedure that a Super-Pro ordered from the factory would be sent out
with its sequentially numbered power supply. However, if one of the many
dealers got in a shipment of Super-Pro receivers and power supplies, he
might not be so careful to assure that a "matched" set be provided to
the purchaser. The matching of receiver and power supply serial numbers
accounting for the use of two sequential numbers from the roster would hold true up to the time that the
HQ-120X was introduced in late 1938. It appears that the HQ-120X used
the same serial number roster as the Super-Pro "sets." After late 1938, the serial number
roster
is split-up by Super-Pro receivers, SP Power Supplies and HQ-120X
receivers. By the time Hammarlund introduced the SP-200, the power
supply serial numbers were no longer sequentially assigned. When
Hammarlund starting to supply Super-Pros for the war effort (probably to expedite production) no effort was taken to assure
that certain power supplies went with specific receivers. It is probable
that specific blocks of numbers from the roster were used for receivers and a sequential
block used for power supplies. This seems to
be confirmed by Super-Pro ASP-1004 SN: 27942 with matching power supply
ASP-84B SN: 28870. A second ASP-84 serial number reported seems to be
too early at 9686 but it may imply that the ASP SNs are a specific
roster of numbers. More ASP data will show whether this is the case. Speakers seem to have an stamped-ink number but it
appears that it may have been identification numbers as sometimes
different speakers
are found with the same numbers stamped inside. For now, we will assume
that speakers were not part of the serial number roster.

This use of sequential serial numbers issued to all
receiver products as they left the line makes estimating production quantities
very difficult. One has to take into account that lower priced
receivers, like the HQ-120X, sold in larger quantities than the
Super-Pro. Also, that the HQ-120X didn't appear until late 1938 and
therefore didn't affect the 1936 to 1938 Super-Pro serial numbers.
Additionally,
the power supply serial number will divide the serial number total by
two for production totals for 1936 through most of 1938. After the HQ-120X appears, a percentage must be
factored into the serial number total to extrapolate the Super-Pro
production.

Without doubt, the SP-10 is a rare receiver.
Very few are ever encountered and from that one naturally concludes that
production was very low. It is likely that no more than 500 SP-10
receivers were built since it was only in production less than nine
months. With the SP-100, production goes from January 1937 up to about
August 1939, or about two years and eight months. Again though, the
SP-100 is rarely encountered. Production should be around 1000 receivers
because the HQ-120X has to be factored into the serial number use for
about the last year of SP-100 production. The SP-150 is so rare with
only 70 produced it is included with the SP-100 group. The pre-war
SP-200 is not a common receiver either. Probably only 1800 Super-Pro
SP-200 receivers were built before WWII began. With the WWII production,
the now commonly seen Super-Pro receivers are found. Production here was
around 10,000 receivers, perhaps more. Total Super-Pro production from 1936 up to 1945
should be around 14,000 receivers. Compare this to the SX-28A production of
around 11,000 receivers (not including SX-28 production) and a
correlation can be seen as to how often any of the pre-war and wartime Super-Pro
receivers are seen
versus how often an SX-28A is encountered. The comparison seems valid.

One note,...the serial number on the military tag of all
BC Super-Pro receivers will not match the Hammarlund factory serial number
stamped on the rear of the chassis. The tag number was usually assigned
by the Signal Corps and has no correlation to the Hammarlund issued
serial number.

The following tables are rough estimates based on
serial numbers encountered weighed against quantity of receivers seen
and the assumed division of the serial number pool.

This is a log of known and reported serial numbers for
Hammarlund Super Pro receivers, their SP power supplies, the HQ-120X
(including military RBH and CHC-46140) and
the Comet Pro receivers. The Super Pros are broken down into the various
civilian models and the three Signal Corps military versions. Likewise,
the power supplies are divided into civilian and military. Be sure to
observe that the Signal Corps tag on the front of the SP receiver has a
Signal Corps serial number and the receiver itself has a Hammarlund
assigned serial number stamped on the rear apron of the chassis. The
Hammarlund assigned serial number is what we are logging.

* = sequential serial numbers - colors match receiver
with ps
** = variation of standard model SP-10 or SP-100 (SP-200 listed as
Special)
*** = matched set with close but not sequential numbers, asterisk colors match
the receiver with ps (Howard) = BC-779 receivers built by Howard Radio Co. as a
contractor during WWII will have "H" suffix in serial number(Special) = SP-200 Receiver with special frequency coverage or
other non-standard features (SC) = Signal Corps - military contract model

Serial Numbers Needed

Since it appears that all Super-Pro receivers and their
power supplies along with the HQ-120X receivers were serialized from a
common serial number roster, any serial number from any of these
Hammarlund products provides important information as to production
quantity and date lines. Even the earlier Comet Pro serial numbers
should show
that it used a different set of numbers. If you have one or more pre-war
Super-Pro receivers, SP power supplies, HQ-120X or Comet Pro receivers, please e-mail the
serial number and what model it goes with to Western Historic Radio
Museum. Be sure if your Super Pro is a military version to send the
Hammarlund serial number stamped on the rear of the chassis not the
serial number on the ID tag . We will add the information to the Known
Serial Number Log. With each number, more knowledge is gained about
these incredible receivers and that information will be added to this
website. Hopefully the result will be an on-line accurate source of information
about the pre-war Super-Pro receivers.

1935 - SPA (SP-10) - June 29, 1935 Order Number
10932-NY-35 issued for Hammarlund SPA Super-Pro Receiver for the Signal
Corps US Army. SPA is identical
to the SP-10 except for the installation of a data plate on the front
panel. It's likely that these receivers were delivered before the
official March 1936
introduction presented in QST magazine that month.

1938 - SP-100 - SP-100L introduced as long wave
version, 100KC-400KC and 2.5-20.0MC, production receivers have same
audio output configurations as the SP-100X.
Late versions of the "L" may have had front panel "Phone" jack, dual
secondary windings - 600Z speaker and Hi-Z phones perhaps
"LX" version added a Crystal Filter
SP-150 console Super-Pro
introduced in July 1938

1942 - SP-200 - Probable beginning of Signal
Corps BC series - some receivers may have rubber stamped SC order numbers
indicating that it was purchased from civilian source.
Militarization of power supplies with heavy duty parts

1943 - SP-200 - Painting of front panel changed
to red oxide primer coat with gray paint, color of the
panels is highly variable with gray, blue-gray and green-gray
commonly seen.
Tube layout charts are added to the top of the RF box
Schematic added to inside of bottom cover of the power supply
S-meter housing changed, full glass front replaced with partial glass
with zero adjust mounted in metal
TM-11-866 Signal Corps Manual, first version. Manual covers BC-779,
BC-794, BC-1004 and R-129/U plus power supplies RA-74, RA-84 and RA-94

1944 - SP-200 - Probable year for contactor built
BC-779

1945 - SP-200 - Late 1945 probable design
date for SP-400 as it is introduced in early 1946

Post -WWII - The Signal Corps continued to
support and use the SP-200 Super Pro receivers. Some of the MWOs and
manual additions date well into the 1950s.
Rectifiers changed in the RA-74D power supply, 5U4 and 5Y3, also 12
cylindrical oil-filled paper capacitors replace the two large block
capacitors (1948)
Improvement Kit MC-531 shown in TM11-866 (Feb 1948 printing) - this is
the three channel selectable crystal oscillator upgrade
Wickes Engineering R-270/FRR receivers supplied for AN/FRR-12 Dual
Diversity Receivers - R-270 is a modified BC-794 receiver (1948)

Expected Super -Pro Performance

The Super-Pro was one of the few double preselection
receivers that was easily available to hams and commercial users in the
mid to late thirties. The
use of two, tuned RF amplifiers on all bands means the Super-Pro was
virtually immune to images and its sensitivity and selectivity were the
best available. Of course, an accessory Pre-Selector could be added to
any single preselection receiver to gain the advantages of double or
triple preselection but the Super-Pro already had two stages on all
bands and the early versions had four IF stages.

When using a Super-Pro the first thing noticed is how
easy it is to over-drive the receiver with either too much RF gain or
too much audio gain. This is especially true of the SP-10 receiver. There is audio power to spare and the sensitivity
can easily be increased to where nothing but noise results. Operating a
Super-Pro is like driving a high horse-power automobile - you don't
drive around with your foot on the floorboard of a Ferarri - the
Super-Pro is the same way,...back the gain controls down and only use what
is necessary.

Another important thing to remember is that the
Super-Pro antenna input Z is approximately 115 ohms. Faraday shields are used in the
first RF coils to keep the impedance fairly constant (only
electromagnetic coupling.) Also, no antenna trim
control is provided. Since this is a relatively low antenna Z, certain
types of antennae work better than others. Random length, end fed wires
are usually a Hi Z antenna and if not matched to the receiver will give
poor results. Almost all commercial users and most hams provided a
matching device for their receivers in the form of the antenna coupler
(nowadays called antenna tuner.) Since the antenna was matched to the
transmitter, which was normally low-Z, the results of using that same
antenna for the receiver worked great. Most complaints about the Super-Pro
and high front-end noise
stem from using a non-matched antenna.

The high power, high fidelity audio provides fantastic
sound when a good speaker is used and it is matched to the particular
audio output Z of the specific Super-Pro. Most SP-10s and nearly all
SP-100s are 8 ohm Z output. SP-100L versions use an audio transformer
with dual secondary windings of 600 ohm Z and Hi-Z phones. A few SP-200s
are dual outputs with 8 ohm Z and Hi-Z phones using frame transformers
similar to the SP-100 transformers but most SP-200s are 600 ohm Z and
Hi-Z phones. If the SP-200 has potted transformers with the dual
secondary windings then it is 600 Ohms Z and Hi-Z phones. The
correct match is important for best fidelity and power. When everything
is correct, shortwave BC stations sound beautiful, especially if
conditions allow for opening the bandwidth up to 16KC. AM BC also sounds
great if you can find a station that is playing music instead of
incessant talking. >>>

>>> Using a 60 foot long end-fed wire
connected to an antenna coupler, I nightly copy ZL stations (New Zealand)
on 40M CW on an SP-200X receiver on loud speaker. The ZLs are Q5 and
about S6. At night (during the winter,) on 80M AM, stations on the East Coast can be easily copied,
providing the QRM allows for it. 20M performance during the afternoon is
incredible with lots of stations from South America and the South
Pacific. DX is a daily or nightly occurrence using the Super-Pro and a
matched antenna. Shortwave BC stations out of South America that are
wide-band sound fantastic since they are strong, rarely fading and the
Super-Pro bandwidth can be opened up to 16KC.

It is unfortunate that these incredible receivers have had
to endure endless deriding from hams and SWLs over the past several
decades. The low opinion of the Super-Pro probably originated from hams and SWLs
who, in the 1960s, happened to obtain a well-used surplus BC-779 receiver (that like
most Super-Pros was still operational) and began using it right away
without doing anything to the receiver. The first thing noticed was that
the highest frequency tuned was 20 MC. Then it was noticed that AM BC
was not included but two long wave bands were. Also, a 600 Z ohm speaker
was needed. Also,
the bandspread only was provided on the top three bands. Complaints were
numerous and mostly based on the surplus BC-779 version. The BC-794 or
any "S" version would have ended almost all of the complaints but it was
one of the most difficult models to find.

The fact that the Super-Pro was built "like a tank" and the RF
box is virtually weather-tight has resulted in many Super-Pros working
in "as found" condition. But, any pre-WWII or war vintage receiver should be rebuilt
and aligned before any sort of critical analysis is to be performed.
What you should expect from your Super-Pro depends upon its condition
and this is true for all receivers, new or old. So, the fore-going
comments are in reference to rebuilt, aligned and unmodified Super-Pro receivers that are
operating at their original specifications. "As-found"
condition will almost always give less than desirable results. Also, make-shift
antennas and non-matched speakers will also result in diminished
performance. When everything is right, the Super-Pro is an unbeatable
receiver, whether it be the SP-10, SP-100 or SP-200, they are all great
performers.

Guide to
Restoring Pre-War and Wartime 'Super-Pro' Receivers

Manuals-Schematics:Though original manuals are nice to own, they are not
really necessary for the repair or restoration of an early Super-Pro
receiver. In fact, most receivers do not exactly agree with the
information in the manual anyway. This is because engineering upgrades
are usually incorporated into production almost immediately while
documentation may take quite awhile to catch up. Most of the information that is in the
various Hammarlund manuals is also
found in Rider's Perpetual Troubleshooter's Manuals. Also, several
sources offer reprints of the original manuals and several on-line
sources offer manuals that can be down loaded.
When ordering a reprinted manual be aware that most manual suppliers
reference the receivers with the Hammarlund ordering model numbers, thus
an SP-100X receiver would be listed as an SP-110X (the SP-200 series may
have the same ordering issues.) Hammarlund did identify the manuals
using the speaker size modifier but since the SP-110X and the SP-120X
are identical receivers, their reasons for this are a mystery. The SP-200 Military versions are covered extensively in the Signal Corps
manual TM-11-866. This manual covers the BC-779, BC-794, BC-1004 and the
R-129/U plus the power supplies RA-74, RA-84 and RA-94. Even if you have
the civilian version SP-200, this TM is an excellent manual to have.
Regarding the SP-10 schematic - the common published schematic for the
SP-10 is for the very earliest receivers. Several engineering upgrades
were added during production which, of course, do not appear on these
early schematics. Hammarlund's documentation apparently never caught up
with the SP-10, therefore, when working on an SP-10, you
should also have an SP-100 schematic handy. You will find discrepancies
in the SP-10 schematic versus most SP-10 receivers that are correct if
you compare the circuit to the later SP-100 schematic. Also, if
you are the owner of an SP-10 manual (hopefully along with the receiver)
note that the manual drawing of the
underside of the chassis is a "mirror image" and shows the RF/IF section where the Audio section
should be and vice-versa - visually confusing.

Reworking Technique and
Skills:Like many of the
pre-war high-end communications receivers, the 'Super-Pro' is a
challenging restoration project. Major disassembly is required to access
some components that need to be replaced or at least tested. How well
your restored 'Super-Pro' functions after a rebuild is dependent on your
soldering technique, your ability to methodically perform mechanical and electronic
work and to keep track of that work. Experienced technicians are always
checking and rechecking their own work as it progresses. This ends up
saving time in the long run as problems at the power up stage are
minimal, if any at all. High-end receivers
cannot be restored in a "rush job" manner. Take your time and recheck
everything you are doing. The SP-10 and SP-100 Super-Pros have more
mechanical complexity than the normal communications receiver and
several of the fiber board parts can be very fragile after years of use.
Don't force assembly or torque anything involving the fiber board parts
or breakage is sure to occur. In a comparison to other receivers, the
'Super-Pro' is just as complicated of a restoration project as the Hallicrafters SX-28. Both require major disassembly because of difficult
(or impossible) to access components that need to be replaced. Remember,
the Super Pro has a rather high level of B+ in its audio section so care
is necessary when doing any testing with the power on. Good
troubleshooting skills are required when working on any type of vacuum
tube electronics gear due to the high voltage levels required for this
type of circuitry.

Disassembly: The best procedure to rebuild
a Super-Pro is to start with a major disassembly. It will be necessary to remove the front panel and some of the assemblies for
access to parts that need to be checked or replaced. There is no way to
access the capacitors in the RF Tuning Unit except by its removal from the
chassis and then removing its shielding. There is no way to remove the RF
Tuning Unit (RF TU) unless the front panel is off. There are five paper wax
capacitors located in the RF TU of the early SP-10 and SP-100
versions and three in the SP-200 versions. The early versions also have
an RF shield between the RF and IF section of the receiver. It is easier
to work in this section of the chassis if the shield is removed. In
fact, it is easier to work on the chassis with the RF TU removed also.
When the capacitors are replaced and the other rework completed, the
receiver can be reassembled. On early models, if there is a problem with
either the "On-Off" switch or the IF gain pot (Sensitivity pot on
SP-100) you have to remove the front panel and the RF TU for access to
remove these components. Check their operation thoroughly while
everything is apart.

Capacitors: All paper wax capacitors
should be replaced for best reliability and performance. Hammarlund did
locate some of the paper-wax caps out-of-sight in the RF TU and in some
of the AVC and Detector transformers but these are easy to access during
disassembly. Whether you use "orange drops" or "yellow jackets" doesn't
matter - both are much better capacitors than the originals were when
they were new and, of course, now the originals have about 70 years of
leakage to deal with. If you want to preserve the under chassis original
appearance, then the new caps should be installed inside the original
shells. This procedure is covered in the section below "Restoring the
100 Series 'Super-Pro.' Restuffing capacitors is, of course, only for
aesthetic purposes. If under chassis appearance is not important, then
install either modern orange drops or yellow jackets. You should use all
of the same kind of caps when rebuilding a receiver. It looks
professional. Mixed types of capacitors look like you were working out
of your junk box! Military 'Super-Pro' receivers will have "bathtub"
capacitors installed around the inside edge of the chassis. These are
multiple paper capacitors installed in a metal box. Some of these are
oil filled units. Also, some of the individual larger paper capacitors
are also oil filled. If the oil filled units are not leaking oil, they
are probably okay. They should be tested at their operating voltage
for possible leakage if you plan on leaving them in the circuit. Or, you can just replace
or rebuild each tub with new capacitors -
your preference.

Paper Wax Capacitor Locations:

SP-10 - five .02uf, 400wvdc paper wax (pw)
capacitors are located inside the RF TU. All other pw caps are under the
chassis.

SP-100 - three .01uf 400vdc and two .02uf 400vdc
pw caps are located inside the RF TU, two .05uf 400vdc and one .02uf
400vdc pw caps are located inside the Amplifier AVC Output transformer,
one .05uf 400vdc pw cap is inside the Detector Output transformer. All
other pw caps are under the chassis.

SP-200 - three .01uf 500vdc pw caps are located inside the RF TU,
four .05uf 500vdc pw caps are located inside AVC transformer, two .05uf
500vdc pw caps
are located inside the Detector transformer. All other pw or bathtub
caps are under the chassis.

Variable
Coupled IF Transformers: You should check the variable
coupled IF transformers for proper action. These have spring-loaded
plungers that are operated by a cam and lever system. The Band
Width/Selectivity shaft is also spring-loaded for a positive feel to the
control. Most 'Super-Pro' receivers require a little bit of adjustment
or rework on this "shaft-cam-lever-plunger" system to have the control
function correctly, feel smooth in operation and to stay in place once
set. A Bandwidth shaft that won't stay in position probably is missing
one of the thrust washers. Add fiber or brass washers as necessary
between the shaft thrust bearing and the rear bearing support. Usually
only one washer is necessary to have the shaft stay in position. The
variable coupled IF transformers require a delicate rework technique to
avoid breaking the fragile parts. I have encountered both broken levers
and broken coil-condenser mounts. Since these are made from fiber board,
they are somewhat fragile. Care has to be taken when removing any of the
variable coupled IF transformers from the chassis since there is an
under the chassis coupling pin between the levers and the plungers. This
is a "slip-in" type of coupling but any torque on the fiber levers will
break the fragile slotted portion. If you have to disassemble one of the
variable coupled IF transformers (to replace a grid lead or for some
other reason) first remove the associated lever from under the chassis.
When removing the shield (can) watch the spring that loads the center
moveable coil mount. The top coil-condenser mount is held in position
when mounted inside the can but when the can is removed there is nothing
except the wire connection keeping the mount on the guide rods. The
spring can lift the upper coil-condenser mount right off of the guides
and launch the spring into the air requiring a search for its
whereabouts. >>>

>>> Sometimes the upper coil-condenser fiber
mount can become "jammed" and break the fragile fiber board holes that
the guide rods pass through. During reassembly if any binding or
tightness is noticed, don't force the assembly. Disassemble the IF
transformer and you will certainly find that the top fiber
coil-condenser mount is "jamming" on the guides and forcing the assembly
would break the guide holes. If the upper and lower condenser mount
screws go in without forcing, then check the plunger operation by hand
from under the chassis. It should travel about one inch and should
operate smoothly without binding. Mount the IF can shield to the chassis
with the two nuts and washers, then check the plunger operation again.
Sometimes the can has to be slightly moved as to how it mounts to the
chassis for smooth plunger operation. Once the plungers all operate
correctly, re-install the levers. They may require a slight offset in the
angle that the lever engages the plunger slot. You can adjust this by
loosening the mounting screw and adjust the lever for the proper angle
and then retighten the screw. Normally, the plungers will rotate to
allow further adjustment of the lever to slot engagement. On the SP-10
and SP-100 versions, the RF-IF shield has cut-out slots that the levers
protrude through. Be sure that when aligning the levers to plungers that
you still have the necessary clearance within the RF-IF shield slots. When everything is
correct, the levers operate the plungers smoothly with no binding or
sticking and with full travel. Generally, if you are careful and watch the levers,
plungers and the
load springs (and nobody else has been into the transformers before you
to break things)
everything will be fine.

Other Components: Resistors should
be checked for drift. Usually 20% is okay. On the early SP-10 and SP-100
receivers there are a couple of what look like adjustable trimmer
capacitors. These are actually fixed caps that are the correct
capacitance when the adjustment screw is tight - no adjustment
necessary.

The dial system is part of
the RF TU assembly but the drive is by a "pinch-wheel" system that
is mounted to the front panel. When
the system is clean the tuning is super smooth. Dirt and grease somehow get in to the
pinch-wheels and will cause a rough feel or even slipping. A good cleaning of
the pinch-wheel and the rim of the dial will usually correct any
problems and result in the super
smooth tuning "feel" that the receiver had when new. >>>

>>> On newer style pinch-wheels there is a nut on
the back of the wheel that should be checked - it should be tightened
when the dial edge is in between the pinch wheels. When reinstalling the RF TU, you will have to loosen or remove the
pinch-wheel drives and then reinstall them after the RF TU is mounted.
That way you can be sure that you don't damage the dials.

If the
Tuning Meter is open on the SP-10 or SP-100 series receivers, there will
be no B+ to the IF section of the receiver. If an original meter can't
be located and the defective one can't be repaired, it is possible to
shunt the defective meter with a 2 ohm .5W resistor to get the
receiver operational until a meter is found.

Front Panels: The front panel of
early 'Super-Pro' receiver is made from .187" thick aluminum. It was
wrinkled finished in black and then engraved so the nomenclature would
appear bright silver. Only the front of the panel is painted - the back
is always left bare aluminum. If the front panel doesn't have its
original wrinkle finish it is next to impossible to restore the panel to
original. Repainting the panel with wrinkle finish will fill in the
engraving and then trying to "scrape out" the paint from the engraved
areas by hand just doesn't work. The nomenclature usually doesn't look
correct or "professional." The wrinkle finished panels were used on the SP-10,
SP-100 and early SP-200 receivers. For best results with an original
finish panel, clean the panel
thoroughly and then touch-up as needed. Using Krylon Black Wrinkle
paint, spray some in a small cup and then paint a thick coat with a
small paint brush where the panel needs to be touched up. Use a heat gun
to force the wrinkle - this will take a few minutes. Don't overheat the
paint or it will "gloss" and not match the original. For very small
areas, artist's acrylic "Mars Black" works quite well and some texture
can be imparted to the touch up with a brush application. If you have an
aluminum panel that is already stripped, paint it with Krylon Black
Wrinkle Finish and after it has dried over-night, try digging the paint
out of the engraving. Maybe it will work okay if the engraving is in
good condition. Many panels that are stripped were done so by "sand
blasting," or "grinding off" the paint. This removes a lot of material
from the panel surface which reduces the depth of the engraving
resulting in the problem usually encountered in trying to remove paint
from the engraved areas. If the panel was chemically stripped you might
be able to successfully restore the wrinkle finish and engraving. If the
results are not to your satisfaction about the only recourse is to paint
the panel with a smooth finish paint the then fill the engraving as
described in the next paragraph. I always encourage restorers to try to
save the original finish if possible. >>>

>>> The .125" steel panels, used on the SP-200 series from about 1941
on, are easy to restore. The panel front was copper-nickel plated and
then a red oxide primer applied before the finish coat was painted. The
back side of the panel is left unfinished. Since the nomenclature is
stamped into the panel, the depression is much wider and somewhat deeper
than engraving. This makes filling in the "lettering" easy after the
finish coat has been applied and has dried. Original paint colors can be
computer matched if some of the original paint is remaining.
Professional automotive paint stores have the ability and the equipment
to not only match the paint but to fill that paint into spray cans for
easy application. Use a high quality acrylic enamel paint that is
semi-gloss finish. Don't use bright white paint or white "lacquer-stix"
to fill in the lettering. The nomenclature will look way too "bright." Instead, mix
artist's acrylic white with raw sienna and just a little yellow to create a
"beige" or "manila" color. This will look correctly "aged" for the fill.
Apply to the lettering one control nomenclature at a time. Let it set
for only a couple of minutes and then wipe with a slightly damp paper
towel folded to be very flat. The "wipe" should be at a slight angle and
only done once with that part of the damp towel. If more wiping is
required, use a new unused section of the damp towel, otherwise you will
get paint on the panel where you don't want it. When all of the
lettering is finished, you can wipe down the entire panel with a clean
slightly damp towel. Let the fill paint dry over-night and then apply
Carnuba wax to the front panel (any non-abrasive wax will work.) Two applications will have the front
panel looking great and with the patina of age imparted by the lettering
not being "bright white."

Cabinets: The early receivers just
use a dust cover. It is black wrinkle finish and is secured with eight
capped thumb nuts on the front and three thumb screws on the back. If
the thumb nuts are missing, they are difficult to find anything like
them today. They can be machined but this is expensive. You can make
them from old nickel plated thumb nuts by filling the top of the hole
with solder, them filing the solder flat. This usually looks okay until
originals are found. The thumb screws are easier to find and usually not
a problem. The SP-200 cabinets will have two chrome strips top and
bottom. These can be missing or damaged. Unfortunately, they are
difficult to replicate since they are thin sheet metal extrusion and
chrome plated. Sometimes a cabinet will be encountered where the strips
and handles were removed and the holes filled and the cabinet repainted.
The military CH-104-A cabinet and the SP-400 cabinet did away with the
chrome strips. The handles didn't change from the SP-200 cabinet to the
early SP-600 cabinet. All are the same. The handles are easy to clean up and
restore. The SP-200 and SP-400 will fit into
either one's cabinet. Early rack mount SP-10 and SP-100 will also fit in
any of the cabinets. Most of the SP-200 cabinets are black wrinkle
finish while most military CH-104 and SP-400 cabinets are gray wrinkle
finish. The SP-600 cabinet is dark brownish-gray wrinkle finish.
>>>

>>> When cleaning these cabinets remember they are durable
and the paint is tough. They can take a lot of abuse without showing it.
That's why I clean the wrinkle finish with Glass Plus and a brass suede
brush. The suede brushes are available at shoe stores or sometimes shoe
sections of regular stores will have them. The brass bristles are not
very stiff and really feel somewhat soft. Get the section you are going
to clean very wet with Glass Plus and then use the Suede brush in
various circular motions, scrubbing the surface. Don't be overly
aggressive but also you don't have to worry about the paint either. You
will notice the wet residue will turn gray-black after a while. Wipe off
the wet with paper towels. The towels will show a lot of color but it is
mostly dirt. Repeat the cleaning until the paper towels are wiping off
fairly clean residue. Perform the cleaning procedure on all of the cabinet
surfaces and then let it dry thoroughly. After drying, do any touch ups
now using either Krylon Black Wrinkle applied with a brush and force
wrinkled with a heat gun (for areas over .5" diameter) or artist's
acrylic for smaller areas or if it is a gray cabinet you're working on.
Apply Armor-All after the touch-ups have dried. You may want to do two
applications. The cabinet will look practically new with this treatment.

Alignment:
Any of the Super-Pro receivers are straight forward with no odd
procedures or special equipment needed. The adjustment of the crystal
filter may require a sweep generator and oscilloscope if the adjustment
has been misaligned, but this is seldom the case. Since the adjustable
465KC IF transformers are over-coupled in the 16KC band width, the IF
should be aligned in the 3KC position at the exact crystal frequency, if
the crystal filter option is installed. Due to the extreme quality that
was built into the tuning unit, all Super-Pros can be aligned to exceed
the dial accuracy specification of 0.5%. If you can't achieve this
accuracy then something
is wrong with the oscillator section for that particular band.

Power Supplies and Power Cables: Early
power supplies used standard can-type electrolytic filter capacitors.
These are almost always dried out and need to be replaced. Even if they
check good, they will certainly fail within a short time if you try to
use them at full voltage. The original cans can be restuffed with new
electrolytic capacitors or the new ones can be installed under the
chassis. For aesthetic reasons, I place the new capacitor inside the
original can. The military power supplies usually have oil filled paper
capacitors for filters. Unless these have shorted or are leaking oil,
they are okay to use. Some of the late version military supplies
will have 12 can filter capacitors installed. These appear to be
electrolytics but they are actually 4uf oil-filled paper capacitors.
They seldom have any problems since they were of very high quality
construction. Test them thoroughly before
relying on them, though. The power supply has two large tapped wire wound resistors
and it is very rare for them to have any problems. Many times the power
supply will not be found with the receiver. They are fairly easy to find
and almost any 'Super-Pro' power supply will work with just about any
'Super Pro' receiver. The power cable is also not usually with the
receiver or power supply. The original cable is not an easy item to find
so most collectors just make one. The only thing to observe is the wires
#1 and #2 are a larger gauge, usually 14 or so, to provide a small IR
drop across the cable for the tube heaters. The remaining wires are
around 18 gauge and not critical as the current carried is low. The
tenth wire, if present, is not used in the receiver circuit. It was a
switched ground for optional user applications.

Modification Mayhem

The Super-Pro is a terrific receiver - whether it be the
SP-10, SP-100 or the SP-200 series - they were the best of the
late-thirties designs. But, they are seventy year old receivers and, as
such, their performance is dated. The stock 'Super-Pro' was an
incredible performer. That is confirmed by the fact that the U.S. Army
Signal Corps versions of the 'Super-Pro' 200 Series are virtually
identical to the civilian versions. The military felt no need to modify
the SP-200 Series unless the receiver was going to be used for data
reception where stability and freedom from frequency drift were of
paramount importance. The military modifications were generally
professionally incorporated and enhance the receiver's performance. The
modifications discussed in this section are the professional "Geisler
mods" and the far more invasive CQ magazine "Surplus
Conversion" modifications.

As the owner of one (or more) of these great receivers, you probably
want to experience what the original owner encountered when using the
receiver when it was new. Something that is accomplished by a thorough
rebuild and alignment of the receiver - not modifications. Understanding
what the designers expected of the receiver and how they intended the
receiver to be operated will help the new owner appreciate the vintage
Super-Pro's capabilities. >>>

>>> Of course, the original owner didn't encounter SSB signals but the Super-Pro will copy
SSB without a problem if operated correctly. Drift, vague dial accuracy,
etc.,...all pre-war communications receivers have the same
characteristics. It's all part of the vintage ham gear experience that
is enjoyed by so many collector-hams today. All serious collectors and
knowledgeable hams agree that major modifications to vintage
radio equipment in an effort to "modernize" its performance seems to go
against the whole idea of collecting, preserving and operating vintage
radio gear in the first place.

Since the early SP-10 and SP-100 Super-Pro receivers
were very expensive and sold during the time of economic recovery, they
are normally not found in extremely modified condition. Most of the
time, repairs and various component changes are all that are
encountered. The unusual way that the SP-10 operates with its highly
flexible front end and adjustable coupling in the Detector and AVC
transformers has led to some SP-10s being modified to operate in a more
conventional manner (rather than the operator "learning" how to use the
receiver.) Most of the mods are to the RF Gain bias although anything
might have been incorporated into SP-10 in an attempt to change its
design characteristics. Most of these modifications are very easy to
remove and return to the original configuration as most were wiring
changes that didn't do any real damage.

L. E. Geisler Modifications - With the cheap, easy to find availability of the surplus
WWII Super-Pro receivers in the mid-fifties and sixties, the
"modification mania" did finally catch up with the line and the WWII
Super-Pro was considered "fair game" for modifications. Most of the
infamous Super-Pro modifications were derived
from the first of the series, "Souping the Super Pro" by L.E. Geisler,
published in the Dec.1957 issue of CQ magazine. Geisler was an engineer
that worked out of Japan for a company that sold modified Super-Pro
receivers. Today, Geisler's modifications are "tame" and basically
replace the 6L7 mixer tube with a different octal mixer tube that is
quieter, then he replaces all of the capacitors and does a full
alignment - pretty much standard stuff. Geisler's earlier mods are conservative,
make sense, improve performance and do no real harm to the receiver. One
has to remember that Geisler's company sold these modified Super-Pro
receivers so they had to perform better yet still retain the
professional-commercial appearance in order to have marketability. As
time went on, Geisler's modification ideas evolved. Later Geisler-modified
Super-Pro receivers will have more invasive modifications.

Post-Geisler Modifications - The later
modification articles go even further than the late-Geisler ideas with
even more and more outrageous modifications. Included in the
list of notorious "cut and hack" articles are "A Super 'Super-Pro'" and
"SSBing the Super Pro" - both published in the "Surplus Conversion
Handbook," part of the CQ Technical Series. These articles
advocate the wholesale modification (destruction) of the entire
receiver, including replacement of the front-end tubes with miniature
tubes, an on-board solid-state power supply, removal of the 14 watt P-P
audio section to install an anemic 6AQ5 single ended audio section
(which also then provided room for the on-board power supply,) on-board
converters to cover 10 and 15 meters, product and infinite impedance
detectors - on and on. It's doubtful that a receiver could ever be
returned to original after being the victim of these later
modifications. I have only seen a couple of Super-Pros that attempted these modifications
and they were
wrecks. No doubt, the end product failed to impress the owners and the receivers
were afterward relegated to the junk pile.

Initially, I thought very few Super-Pro receivers had
ever been modified since it was such a great design in the first place.
The Super-Pro receivers that I had found around Nevada were mostly
all original. However, after talking to several collectors in different
parts of the country, it seems that there are lots of modified Super-Pros out there.
Many modified versions are now showing up on eBay. Certainly, the rarity
of the particular receiver will dictate how far the owner is willing to
go to restore the set to its original specifications. In a way, it is
fortunate that the
most common victims of the modification mayhem are the military receivers.
They are also the
most plentiful version around and that means that most of the SP-200 parts are
usually easily
available to complete a "restoration to original" on other SP-200
receivers that have faired better. However, a true "Geisler Modification" or
a "Military Modification" receiver should be left "as-is" since these
modifications made sense and enhanced the Super-Pro performance.

Using the Super-Pro as a
Communications Receiver Today

Some hams are reluctant to use a pre-war receiver in
actual "on-the-air" operations for fear that adjacent frequency QRM will limit their ability to successfully copy
stations and that they will be unable to complete QSOs or Vintage Net
operations. The Super-Pro might have problems today coping with adjacent
frequency interference from powerful SSB signals when the user is trying
to copy an AM signal (and this will be the case for almost all vintage
communications receivers.) Even narrowing the "Bandwidth" IF passband or
using the crystal filter doesn't seem to help much. However, most of the
problem is caused by the user operating the receiver as a
"broadcast radio" rather than as a "communications receiver." Most manuals
will direct users to operate the receiver in the
"standard AM mode," that is with AVC on, the "Sensitivity" at
maximum and the Audio Gain rather low. This allows the receiver's front end gain to be controlled by
the AVC in response to whatever the strongest "signal" happens to be.
This works fine when listening to strong broadcast signals. But many times the
desired signals are weak and the stronger energy will be atmospheric RF noise.
Often times it is
adjacent frequency interference. The best solution is to switch off the AVC,
increase the Audio Gain to near maximum and operate the receiver's front
end gain manually with the "Sensitivity" reduced to the minimum
necessary to copy the desired AM signal. You will find that now the
adjacent frequency interference is
greatly reduced since its strength compared to the tuned signal is
reduced. Also, the AVC-controlled gain is no longer responding to
variations in what it "sees" as a signal - the "pulsating" and "popping"
of adjacent frequency signals within the passband. >>>

>>> Additional
selectivity using either the variable "Bandwidth" control set at 3kc
and/or the crystal filter will also help to reduce the ratio of adjacent
frequency
interference to tuned signal. You will have to tune the desired signal
"on the nose" for good copy. A couple of losses in this method of
operation will be the S-meter function but it is really not very useful
and only provides a relative measurement anyway. Also, broad audio
fidelity cannot be enjoyed in this mode. But, the goal is successful
copy and a completed AM QSO, in other words - communications. What about
in actual use? I use only vintage receivers for operations on the ham
bands and many times I'll use the SP-100X version of the Super-Pro as
the station receiver. Using the "communications receiver" method of
operation has allowed me successful copy of all stations checking into
the vintage AM nets regardless of the adjacent frequency QRM. Weak signals can be a challenge but switching in the
crystal filter usually provides the extra edge necessary for successful
copy. Of course, it isn't enjoyable, "arm-chair" copy but it is a
successful completion of the QSO or of the net operations, which is the
goal. Unfortunately, deliberate on-frequency interference is another matter and no
receiver, no matter how sophisticated the design, can cope with that
type of intentional QRM.

Restoring Pre-War Super-Pro
Receivers

The WMI - Hammarlund 'Super-Pro' SP-10
Restoration

I found SP-10 SN: 576 listed on QTH.COM. in January
2008. Its purchase included an original SP-600 cabinet thrown-in.
Unfortunately, shipping ran about 55% of what the price of the receiver
and cabinet was but it was safely packed and arrived without any issues.
Initial examination revealed that the condition was very good
cosmetically but the under chassis was almost totally non-original. I
knew the receiver had been used extensively at WMI and had seen detailed
photos of the underside so this was no surprise. I knew what I was going
to be getting into.

photo
left: The WMI SP-10 before restoration

At first, I thought I might be satisfied with the SP-10 given its WMI
provenance but closer examinations kept revealing more and more late,
post-WMI components and mods. I noted that almost all of the installed
capacitors were Vitamin-Q types date coded 1973 (about ten years after
the receiver left WMI.) I came to the conclusion that about half of the
receiver circuitry modification was not WMI's work and that only the
visible front panel mods were the remains of WMI "hacking."

Our decision was to return SP-10 SN:576 to "original - as
delivered to WMI" in 1936. Since any WMI provenance had been compromised
by forty years of post-WMI modifications there was no justification to
the belief that the receiver was in "as used at WMI" condition. By
restoring SN:576 to original, the Hammarlund historical accuracy for the
SP-10 Super-Pro model would be returned to this rare receiver and, at
the same time, the WMI provenance would be enhanced by having the
receiver look and operate as it did when WMI took delivery.

This was going to be a thorough, "ultimate museum
quality" restoration using rebuilt capacitors in original paper-wax
shells, original type resistors where needed and authentic Hammarlund
parts from other Super-Pro parts sets. We even used some of the original
Hammarlund wire from the parts sets. We were going to strictly adhere
the original Hammarlund wiring layout and to match exactly the original
circuit design - taking into account that there were two original
Hammarlund engineering upgrades not on the SP-10 schematic that had to
be left in place. During the restoration, every step was guided by the
desire to have this SP-10 be the most authentic, best representative of
how the Hammarlund SP-10 appeared and functioned when it was new in
1936.

To guide us, we used our experience gained during our
"museum quality" restoration of the SP-100X SN:3387 receiver along with
the evidence found during the SP-10 disassembly, the under chassis
artwork in the SP-10 manual and the SP-10 and SP-100 schematics
as references. Hopefully, this would result in a very authentic,
restored SP-10 that could be used as a reference for how these receivers
were built, how they performed and how they looked - both externally and
internally.

The Planned Restoration Work

I had never encountered a receiver that had so
many deviations from original wiring layout, in component placement or
component type but that was still functional and still looked decent.
This was primarily due to the WMI repairs done over the years. Since the
receivers were used 24/7, repairs must have been a somewhat regular
necessity. Originality was not a concern and neither was neatness. The
end result of years of use and repair cycles is a receiver with very few
original parts, several wrong value parts and many modifications to
component placement or layout with some circuitry modifications thrown
in. A lot of later post-WMI work was also performed on the receiver,
also with little regard for originality. Here is the list of the
deviations from original.

P. Shielded cable from detector to BFO - non-original cable (modern)
- original may have deteriorated

Q. BFO grid cap was 500pf, should be 100pf - repair with wrong value
part - may have been WWII repair when parts not available

R. Screen load on 2RF amp was 2.4K should be 5K - repair with wrong
value part - may have been WWII repair when parts not available

S. 220 ohm resistor added to B+ line to CT of audio output xmfr plate
winding - to lower B+ from +385 to +365vdc at the 42s - allows using
SP-100 PS

T. Though minor, someone wrote in black marker ink all of the circuit
functions on the inside of the chassis. Additionally, dymo-lables were
on the RF box indicating alignment functions. Also, all of the IF and
AVC transformer cans were marked with marker ink. - Who knows why?
Probably someone thought it would save time in troubleshooting if the
circuit and component functions were obvious.

Planning and Finding Parts:I waited about
2 months before starting the restoration. This gave me time to do
research, studying and planning of how to accomplish the rework. I was
going to need 35 paper-wax capacitors to build the replica capacitors.
The original paper-wax caps had been replaced eons ago but I wondered if
the "hackers" had ever gotten into the RF box. When disassembled, I
found three of the five capacitors were indeed originals. They were
Aerovox brand, which meant I would have difficulty in finding enough
correct vintage shells. After searching only turned up five or six
correct style Aerovox shells, I decided I might have to go to Cornell-Dubilier
TIGER brand, as they were standard originals for the SP-100 and later
receivers. Fortunately, in searching around I made an important
discovery. While looking at a junker HQ-120 receiver to harvest C-D
caps, I noticed that the original capacitors were intermixed Aerovox and
C-D brands. I was positive all the capacitors were original. I had
discovered a precedent that Hammarlund did intermix capacitor brands
sometimes, so I would do the same thing with the SP-10. Five Aerovox
shells would be used where the caps mount on the tube sockets, three
original Aerovox shells would be used in the RF box and all the other
capacitors would be Cornell-Dubilier TIGER brand.

The Importance of Having Parts Sets: I had serious
doubt that the SP-10 dial windows with index line were original. They
were a crudely made lamination of plastic and the scribed index wasn't
even straight. I convinced myself of their non-originality when I pulled
apart the laminate and the glue was still somewhat tacky. Luckily, I had
an SP-200 parts set. This poor receiver has served its fate as a donor
for three different Super-Pro restorations and it was nearing its end as
a useful "parts set" since there were very few parts left on the
chassis. But, even though most of the parts were missing, this receiver
supplied dial masks, dials, both dial window indexes, AC power harness,
IF transformer parts, correct dial lamp harness, a multitude of
screws and washers, some knobs, some wire and some meter parts.
Additionally, an HQ-120 junker supplied many Aerovox and C-D capacitors.
The junk box supplied a correct style fiber tube socket with the correct
"42" ID, several of the correct style/value resistors. Without these
junk sets and several junk boxes to rob parts from I doubt that the
SP-10 restoration would have ever started since correct authentic
vintage parts are so difficult to find.

The SP-10 Restoration Work

The photo to the left shows the front panel before restoration. The
physical modifications were going to be difficult to repair. Most of the
time, amateur modifications have little regard for symmetrical layout or
quality machine work. The WMI SP-10 mods, even though performed by
"professionals," were very much like amateur workmanship. I knew from an
earlier restoration of an SP-200LX receiver with a repainted aluminum
front panel that stripping and repainting wrinkle finish on this
aluminum panel would be a disaster. Trying to remove the paint from the
engraving is next to impossible. Touch-up is the only method that
preserves the original look of the receiver panel. Filling the holes
presented a challenge because of their size. First, the panel was
thoroughly cleaned using Glass Plus and a brass "suede" brush. This is
done to clean the smoke and grime out of the engraving so it would look
silver again. The only practical hole repair method was an epoxy fill. I
placed masking tape over the holes on the front side of the panel and
then backed that with blocks of wood clamped to the panel. This gave a
very flat surface and the masking tape assured that the epoxy would not
stick to the backing blocks. I fill from the back side using slow curing
epoxy to make sure all of the bubbles have a chance to rise to the
surface (the back of the panel.) Because of the thickness of the panel,
two layers of epoxy were required for the complete plug. The epoxy was
left to cure overnight and then the blocks and tape were removed. This
leaves a super-flat surface on the front of the panel. The back side is
leveled using a Dremel Tool. The front side is painted using Krylon
Black Wrinkle finish applied with a brush and "wrinkled" using a heat
gun. Only the plug is painted and if everything goes okay, the match is
virtually undetectable. The backside was painted with silver paint.

The photo to the left is a shot of the chassis underside
before restoration. There was so much that was non-original I created a
list to keep track. Some of the notable deviations under the chassis -
the shielded cables routed next to the RF box. These carried the audio
output to the phone jacks mounted on the front panel. The transformer
matched the 8 ohm Z output of the receiver to the 600 ohms required at
WMI. None of the capacitors are original, most having been installed in
the 1970s after the receiver's tenure at WMI. Many modern style
components were installed in the various circuits.

The photo to the
right shows the SP-10 underside after restoration to original. The
wiring layout follows the under chassis drawing in the SP-10 manual and
also required several references to the SP-100 receiver. This was to
assure that all wiring and component placement matched what was done at
Hammarlund.
Note: The shielded cable most visible next to the RF box carries the AC
power to the front panel switch. This is an original Hammarlund harness
that was removed from a parts set and was installed during restoration.

The close-up photo to the left shows the AVC and BFO sections with
rebuilt capacitors and replica resistors. These rebuilt capacitors
happen to all be Cornell-Dubilier TIGER brand. This receiver restoration
was different in that I had to find all of the shells first. This
allowed me to rebuild the caps ahead of actually installing them. This
seemed to make the job go much faster because I could do several
capacitors at one time. Even so, the time per capacitor still was about
ten minutes. My method is to use a heat gun to melt out the original
capacitor and then wipe the shell with a paper towel to clean it. I then
install a film capacitor of the correct value and correctly oriented and
secure it with hot melt glue. After the glue has set-up, I fill each end
with brown sealing wax. The end result is a capacitor that is new but
difficult to distinguish from an original.

The two 60K resistors are replicas that are repainted to match the BED
coded originals. There were several JAN type resistors installed and
even some modern metal-film resistors that were removed. To keep
everything looking the correct vintage I used similar to original size
and shape resistors of the correct value. These similar style resistors
were marked in banded color codes. Also, some of the replacement
resistors found had leads that were too short for the Hammarlund
"thru-the-eyelet" type of mounting. I had to carefully add a length of
TC wire to be able to mount the resistors correctly. When the leads were
installed, I then painted the body of the resistor in the BED code
style. When mounted, only a close inspection reveals that these are
replica resistors.

I initially installed the three IF screen bypass
capacitors as the replacement, non-original caps had been installed - on
top of the component board. This had the capacitors on top of the 10K
resistors. This didn't seem likely and I remembered that in the SP-100
rebuild, the IF screen caps had been mounted at the tube sockets.
Looking at the manual drawing, it was apparent that the caps shouldn't
have been mounted on the component board. I removed them and moved the
component boards in order to have access to the IF amp tubes. There I
found the evidence I was looking for - the actual remains of the
original capacitor leads left behind when the receiver was re-capped
years ago. I installed the rebuilt caps at the tube sockets, as was
original manufacture. The photo to the right shows the RF-IF section
with all rebuilt capacitors installed in their correct positions. The
Aerovox rebuilds installed in the RF section can be seen. The LO
capacitor that looks like an adjustment trimmer is actually a fixed
capacitor that is the correct value when the screw is tight. Though this
one is a replica I made, it is very close to the original in size, value
and appearance.

The photo to the left shows the bandspread condenser side of the RF
Tuning Unit and the rebuilt paper-wax capacitors. These five capacitors
are completely hidden and require the removal of the RF TU from the
chassis and then the removal of the side covers to access the capacitors
for replacement. The photo to the right shows the replacement fiber tube
socket (the one on the left) that was installed to replace an incorrect
bakelite socket that had been installed in a repair done many years
before. Luckily, I found an exact style fiber socket with the correct
"42" tube ID in one of my junk boxes. Nowadays, we probably shouldn't
call them "junk" boxes since the parts they contain are so necessary for
restorations and are so difficult to find otherwise.

None of the grid
leads were the correct, rubber insulated wire. The correct color is
cream. I found that the old style large gauge round AC power cables use
rubber insulated wire and the neutral wire is a creamy white color
rubber insulation material that provided a good match for the original
grid leads. Fortunately all of the grid caps were original and were
reused for the new grid leads.

In addition to the rubber insulated grid leads, there were two
shielded cables that needed to be rebuilt. The output cable from the
Detector to the First Audio stage had deteriorated and was on the verge
of shorting. By using the same rubber insulated wire I was able to "push
thru" a new center conductor that looked just like the original. A
replica shielded cable had to be made for the BFO output to the Detector
cable. This had been a piece of modern phono cable which we replaced
with a replica that matched the original style.

photo above: the variable coupled IF system levers. The
top one is good, the bottom one is broken

Rebuilding the Variable Coupled IF Transformer
System: I noted that the Selectivity control shaft did not
stay where set and that the action felt very loose. Inspection revealed
that two of the three levers were broken and were not even moving the IF
plungers. Removal of the variable-coupled IF transformers showed that
earlier repairs had broken the fiber board guide holes on two of the
transformers. The parts set provided the necessary replacement parts
including two good condition levers. See the section "Guide to Restoring
Super-Pro Receivers" in this web article for more details on general
rebuilding of the variable coupled IF section.

Tuning Meter: The Tuning Meter has a more modern movement
that replaced an open coil original. The case and glass are
original. It was necessary to install a shunt inside the meter case so
that the meter would have the proper range. This had to be selected
after the receiver was operational since there are no specs for what the
original meter movement was. Testing showed that 7.0mA fs gave the best
action and range. If I can ever find an original functional meter for
either an SP-10 or SP-100, I will replace this meter since it is not
totally original.

Replica Dust Cover: The original dust
cover had been discarded many years ago by WMI. I tested the fit of the
dust cover from my SP-100 and found that it fit perfectly. I bought some
20 ga. sheet metal and did the layout for the dust cover. I carefully
cut out all of the vent holes and marked all of the bends necessary. I
had a local sheet metal shop do the bending and spot welding. The
replica fit perfectly when I got it back from the shop. Next, I had to
make the studs out of 8-32 threaded stock. I didn't have the necessary
small rivets used originally to hold the studs in place. Instead, I used
2-56 screws and nuts to secure the studs. I did modify the screw head to
look like a rivet. When installed only the nuts inside give away the
fact that these are replicas. I was lucky that fellow ham KØDWC had some
of the original 8-32 cap nuts and I made the rear 6-32 thumb screws.
Painting was all that was left. The inside was painted gloss black and
the outside painted with Krylon Black Wrinkle finish that is "baked" to
force the wrinkle. The finished cover is next to impossible to
distinguish from the original, except that we don't have the rear ID
plate - but we did drill the holes so it would look as if one was there
at one time.

Final Testing and Alignment: I modified a late
manufacture SP-100
power supply to use with this SP-10. By installing a 250 ohm 5 watt
resistor between the +385vdc supply and the PS output terminal, the
voltage is dropped to about +360vdc at the SP-10's P-P 42 audio output
tubes. The other voltages are the proper level for an SP-10. The late
SP-100 PS was the one that came with the SP-10 and probably had been
with the receiver for a considerable period of time. Being a late build,
this PS had a filter choke installed to sub for the speaker field which
allows the use of a PM speaker. I also was
using an SP-200 power cable which has larger tube heater wires for a
smaller IR drop across the cable. Upon power up there was no
signal, just audio hiss. Within a few minutes the smell of "hot
resistor" was noted and the power was shut down. The Mixer plate load
resistor had gotten very hot but what caused it was unusual. The wiring
for the tube heaters in the SP-10 is unconventional in that some of the
heater wires pass over the tube sockets instead of around them. In this
case the plate pin of the Mixer tube was contacting the heater wire and
the insulation was thin or gone - anyway, a short occurred and caused
the hot resistor. I moved the wire and then applied a paint on
insulation (black) to assure the problem didn't happen again. Power was
again applied and this time the SP-10 burst into a wonderful audio sound
experience. I was amazed by the sound quality of the AM BC station
that happened to be tuned in. Unbelievably wide range, bass-laden music.
I was impressed. Not that there weren't some minor issues, though. I
still had to set the shunt in the Tuning Meter and perform a full
alignment. After that, the audio from the SP-10 is just fabulous. It
sounds very bassy and wide range when receiving AM BC or SW BC when
conditions allow for great reception of the South America SW stations
that play music. AM hams that run some power sound incredible. The
entire operation of the receiver is exactly as described in the SP-10
manual and certainly a pleasure to listen to. Trying to imagine what an
original owner must have thought of his new Super-Pro in 1936 is always
interesting - too bad they were so expensive that few hams could afford them
in 1936. Certainly signals are quite different today, but still it must
have been thrilling to receive shortwave stations from around the world
on a then new Super-Pro.

Restoring the 100 Series "Super-Pro"

I owned this 1937 SP-100 for about four years before I decided to
restore it. It was an e-Bay purchase that happened to have been offered
by a seller that was located only about 25 miles away. I e-mailed,
asking if I could come over and look at the receiver before I bid on it.
The seller was more than happy to agree so I drove down to Gardnerville,
Nevada to have a look. The SP-100 was in good physical condition and was
complete with the matching serialized power supply. I bid on it and won.
So, with another trip to Gardnerville, I became the owner of this great
receiver. I didn't expect it to work and a quick check over found
several things that needed to be repaired before it was powered it up. I
only did "quick fixes," just to see how the receiver would
perform. I used the receiver a few times
but never trusted it with long operating stints. I had planned to
restore it long before I actually did - but delays on projects seem to
be the norm around here.

At the end of 2007 I finally got some time to do this SP-100. A detailed
inspection of the chassis showed that many capacitors had been replaced
over the years - mostly using Sprague molded caps similar to "Black
Beauties" but without the color-code stripes. Some of the resistors had
also been replaced since they had burned up when the original associated
capacitor failed. All of the other parts,...IF transformers, AF
transformers, the meter, etc. were all original and in good operational
condition. I wanted to perform a "museum quality restoration" on this
SP-100 as it was an excellent example of this rare receiver. Our "museum
quality restoration" results in a fully functional receiver using the
original design circuit with the entire appearance of the receiver as
close to original as possible with the patina of age preserved. The
under chassis appearance has to look original, therefore all capacitors
are "re-stuffed" with new film caps inside the original capacitor shell.
Any resistors that are replacements have to be the original style part.
Any defective parts are rebuilt and if that is not possible, a correct
style and manufacturer part is used as a replacement. When the rebuild
is completed, the receiver is fully tested and aligned. The completed
receiver can be used as a reference, illustrating how the originals
looked - on both the exterior and the interior of the set. Also, I had
wanted to document the performance of this
receiver, so it was
necessary for it to function reliably at its design limits.

photo right: the restored SP-100 chassis - note
the differences in this chassis and the SP-10 chassis. The lack of
adjusters on the Det and AVC transformers, the metal octal tubes used
and the different style audio transformers.

photo above: The bandspread
condenser side of the RF box showing the "hidden" paper-wax caps inside.
Note the Isolantite material used for the coil mounts and the variable
condenser mounts. This was a low loss ceramic material.

Rebuilding Capacitors

When checking the schematic, the parts list shows that
35 paper-wax capacitors are used in the SP-100 circuit. But
comparing that information with what can be seen under the chassis, it
becomes apparent that nine capacitors seem to be missing. They
aren't - they are located inside the RF box, inside the 2nd Detector
Output Transformer and inside the Amplified AVC Output Transformer. The
RF box caps are difficult to see let alone replace. Disassembly of the RF box is necessary to
have easy access these five capacitors. Unfortunately, you
can't just remove the side covers - you have to remove the entire RF box
from the chassis first. This isn't as difficult as it sounds - eight
wires must be disconnected, the front panel removed and 10 mounting screws taken out to remove the RF
box.

photo above: The
completed RF box fully assembled and ready to install. Note the new grid
leads and grommets. There are 33 screws for the bottom shield, 20 screws
for the two top covers and 8 for the back covers along with the 50
screws for both side covers. Total of 111 screws just to hold the shields
and covers in place.

There are also three paper-wax capacitors inside the
Amplified AVC Output Transformer. In the photo to the left the .02uf and
one of the .05uf caps are visible. The other .05uf cap is on the
backside of the fiberboard mount.

There is one remaining paper-wax capacitor inside the 2nd Detector
Output Transformer. It is a .05uf shown in the photo to the right. What
appears to be trimmer capacitors are actually an assembled fixed
capacitor. There is one on the back of the board also. When the screw is
tight, the capacitance is at the required value. These are
original and are Hammarlund parts. Behind the board was a 5K ohm
resistor that was burned and measured 1K ohm. This was replaced with a
correct vintage part.

Also shown in the photo to the right is the deplorable condition of
the grid leads. More on this problem below.

Since more than half of the original paper-wax capacitors had been
replaced in the past with plastic molded style caps, I had to locate 18
Cornell-Dubilier "Tiger" paper-wax capacitor shells with the correct
values to build my restored caps. I had an old SP-200 parts set that
became the "donor" for these correct capacitor shells. I use a heat gun
to melt out the old original cap leaving just the shell. I wipe the
excess wax off while the shell is still hot to clean the surface. I then
install a new metalized-film capacitor of the correct value inside the
shell. I orient the caps all in the same direction with regard to the
outer film marker on the shell though it really doesn't matter with
modern film
caps. I secure the new cap in place with hot melt glue and when that has
cooled enough, I fill each end with brown colored sealing wax. The whole
process takes about 10 minutes per capacitor. The results are shown in all
of the under chassis photos - all of the paper-wax capacitors
shown have been rebuilt. I install the rebuilt caps in the proper
direction. This whole process is for cosmetics, it does nothing to help
performance. If under the chassis appearance is not important, then just
install the correct value, modern capacitor.

It is easier to work on the SP-100 chassis if the RF box is removed -
you have to do this anyway to replace the five capacitors located
inside. Also it is easier to work on the RF/IF section if the shield
between those two sections is removed. The photo to the right shows the
RF/IF area of the chassis with the shield removed. Also to access the
bypass caps on the IF amplifier tubes it is easier if the small
fiberboard component mounts
are placed out of the way by removing the mounting screws and a few of
the wires. This allows open access to all of the parts that need
replacement. There are nine of these small fiberboard component mounts
under the chassis but only the three over the IF amplifier tubes need to
be moved.

photo above: The Crystal Filter assembly with new
grid lead and new connecting wire for the Phasing Condenser and Crystal.

Other Restoration Work

Once all of the capacitors were rebuilt, it was necessary
to replace all of the grid leads from all of the IF transformers and AVC
Output Transformer and the BFO coil. The Crystal Filter assembly is
rather complicated in its construction and was removed from the chassis
in order to easily disassemble, replace the grid lead and the connecting
leads to the crystal and the phasing condenser and then reassemble.
These grid leads were originally rubber insulated stranded wire but the
rubber had become "lumpy" and had hardened, becoming brittle. Any flexing
would break the rubber off of the wire. I used a cream colored cable
jacket that was off of old telephone hook-up cable. I stripped the outer
jack off and then inserted a stranded wire into the jacket to build grid
leads that had the correct feel and looked pretty close to the original.
I was able to reuse all of the original grid caps. All new rubber
grommets were installed also.

All resistors were checked for value and all were found
to be within 20% of the correct value except the burned resistor in the
2nd Detector Output Transformer.

While most of the assemblies are off of the chassis is a
good time to clean the chassis. I just used Glass Plus and a horsehair brush since
the chassis was in good condition. Also, this is a good time to
thoroughly check the Sensitivity potentiometer. This part cannot be
removed when the RF box is installed. In fact, replacement of this part
normally requires removal of the front panel and the RF box to
accomplish, so now is the time to check it (this is also true of the
"ON-OFF" toggle switch.) I disassembled the Sensitivity pot and cleaned
it but it was going to become a future problem after re-assembly.

photo above: The underside
of the chassis complete except for the 33 screws that hold the bottom
plate on the RF box. Also note that there is absolutely no clearance
behind the Sensitivity pot if it needs to be removed. Same goes for the
"ON-OFF" switch.

At this point the receiver was ready to reassemble. When
replacing the RF box, the two pinch wheel drive housings have to be
loosened and then the two dial edges guided in between the drive wheels as
the RF box is placed on the chassis. Once the dial edges are engaged then the pinch wheel
housing can be retightened and the dial drives tested. There shouldn't
be any slipping and the drive should be ultra-smooth. Then the screws
that hold the RF box can be tightened. The idler gear for the dial mask
drive needed to be mounted and adjusted - the assembly can be moved
vertically for centering the dial mask and then the screws tightened.
When the front panel is bolted in place then the Crystal Filter panel
can be mounted followed by all of the knobs and the tuning meter. I
tested all of the tubes and found them to test fine - at least in the
tube tester. The receiver was now ready to test and align. I had a couple of problems
turn up after a short period of operation. First was a noisy 6B7 tube in the detector
stage. This showed up as a continual but erratic "rushing-thumping"
noise that varied with the AF Gain control. Second was a "noisy" 6F6 in
the push-pull AF stage. This showed up as soft, weird noises (erratic
audio oscillating) that was present even when the AF Gain was reduced to zero.
I guess this shows that even the best tube testers don't catch
everything.

During the alignment another problem showed
up. Audio distortion was noticeable while in AVC and the Sensitivity
control didn't reduce the RF/IF gain when the receiver was in AVC. This
problem was caused by a bad solder joint in the AVC line to the RF
amplifiers and an intermittent Sensitivity pot that ultimately had to be
replaced. The finished SP-100X has
fabulous audio with plenty of power, formidable bass and a very wide
audio response when in the 16KC IF bandwidth. Excellent dial accuracy -
easily better than the 0.5% specification. Sensitivity is at the limits
of what antenna noise is present and selectivity is sharp in the 3KC
bandwidth and ultra sharp with the Crystal Filter. AM-BC and SW-BC
stations sound incredible. Vintage AM Ham stations that run some power
(like retired AM BC transmitters) are a pleasure to listen to. Simply a
great receiver.

photo above: The finished
1937 SP-100X sn: 3387 (ps is sn: 3388)

Competition Comparisons

The Super-Pro was introduced in March 1936. What was the competition at
the time? There was the famous National HRO, a great performing receiver
at a slightly lower price than the Super-Pro. The RME-69 was available and by
November you could also buy the DB-20 preselector - for a package price
about the same as the HRO. Hallicrafters was
offering an inexpensive and popular receiver in the contractor-built
SX-9. Patterson offered receivers that were built at the Gilfillan Bros.
plant in Los Angeles, California. RCA had the AR-60
Professional Communications Receiver but at nearly $500 nobody bought
them and consequently only a few hundred were built. The following are
some details on the best of the 1936 competition.

The National HRO

The HRO was an incredible receiver, especially when it
came to its sensitivity and almost unlimited bandspread. The price of a
new HRO was usually around $200 with a set of four coils, power supply
and a speaker. If the new owner learned to use his HRO, he could get
impressive results with the receiver. But, there were several odd things about
the HRO. First were the plug-in coil assemblies that made
changing bands a
hassle. Even storage of the unused coil sets could be a problem. If general
coverage was desired (for example, to check shortwave broadcasts) the owner had
to move four small screws on the coil set and screw them into different
locations. Then remove and replace the screws again to go back to bandspread.
Not to mention that the coil assembly had to be removed from the
receiver each time to change screw positions. It was such a hassle, that most hams just forgot about using
the HRO for anything other than a "ham band" receiver. Then there was
the micrometer dial,...elegant in design and super smooth to operate but
the new owner had to read the dial then correlate that to a graph on the coil set to determine
tuned frequency. AM BC coverage required the purchase of two more plug in coil
sets. With all of these quirks, why was the HRO so popular? Because
it delivered absolutely the quietest front-end performance meaning that
weak DX signals were easy to copy. Also, the ham bandspread was endless
- equivalent to a linear dial over nine feet long! If you wanted a receiver
for performance first and convenience last, the HRO was for you.
Compared to the Super-Pro, the similarity is of course the front-end
with double-preselection. The HRO will have the edge on greatest useable
sensitivity but not by too much. Setup for bandspread, the HRO is
unbeatable for tuning resolution. However, the variable Selectivity of the Super-Pro,
its direct readout dial accuracy and its
powerhouse audio are its definite advantages over the HRO.

photo above: 1935 National Company HRO
Receiver

The RME-69/DB-20

The RME-69 was a popular
receiver with hams. It wasn't too expensive (at first) and delivered
great performance when used with a pre-selector. So much depended on the
pre-selector that finally, in November 1936, RME provided one of their
own, the DB-20. This addition gave the user three tuned RF amplifiers,
providing top-notch sensitivity and image rejection. The tuning and
bandspread dial drives are velvet-smooth with plenty of vernier reduction.
The dial illumination is great - it has to be since the dial
nomenclature is so small it can hardly be read without magnification.
Six tuning ranges gave the user coverage from .54 up to 32MC. The power
supply was built-in. About the
only accessory needed was the speaker. The RME-69/DB-20 was a great
performing combo but it had its quirks. There are no trimmers for the RF
or Mixer coils, a Compensator control keeps those stages peaked for any
tuned frequency. But, it's very easy to tune the Compensator to an image
on the higher frequencies - a disaster for correct front-end tracking.
The standby function is accomplished by pulling out the Audio Gain
knob/shaft but there is no remote standby provided. Some of the
construction is a little on the cheap side - especially the compression
trimmers provided for the LO alignment. All in all though, the great
performance made up for any shortcomings and the RME-69/DB-20 gave the
hams a relatively compact receiver packed with lots of features. Selling
price was about $200 when the DB-20 and speaker were included.
Compared to the Super-Pro, the RME-69/DB-20 combo will perform as well on sensitivity but
the continuously adjustable bandwidth and the 14 watts of audio power is the Super-Pro's
advantage.

photo above: 1937 RME-69 Receiver, DB-20 Preselector, RME
speaker

Patterson PR-16C

The PR-16C was designed by
Karl Pierson and is mostly remembered for using parallel-connected RF
amplifier tubes. The argument was that parallel tubes doubled the
transconductance and that increased gain and lowered the thermal noise. It also reduced the
grid input Z but nobody (except James Millen of National Co.) seemed to
realize that. Additionally, the PR-16C was advertised as having two RF
amplifier tubes - Patterson may have even mentioned that the tubes were in
parallel but didn't elaborate that only one set of RF coils per band
were used and that the PR-16C was really just a single-preselection
receiver and was just as prone to image problems as any other single- preselection receiver. The other
notable feature in the PR-16C was the incredible
audio amplifier - three stages of push-pull audio amplification. A 6A6
dual triode is transformer coupled to two 76 triodes that are in-turn
transformer coupled to a pair of triode connected 42s. The audio power
was rated at 18 watts and the receiver actually sounds incredible when
driving a high quality speaker. The PR-16C also had two-speed tuning,
band-in-use masking, chrome plated chassis and, of course, 16 tubes.
Really, a lot of features for the list price of $101 - but its overall
performance as a communications receiver couldn't come close to the
double-preselection receivers,...the Super-Pro and the National HRO, or
the RME-69/DB-20 combo. The PR-16C's audio section is certainly
comparable to the Super-Pro, if fact, it even has more power. The PR-16C
is an incredible audio machine but is somewhat lacking at the RF/IF
section.

photo above: 1936 Patterson PR-16C

Hallicrafters SX-9

This was Hallicrafters' last offering of a receiver that built by
contractors. After the August 1936, all Hallicrafters' receivers were built by
Hallicrafters' assemblers at their new plant that had been purchased from the Echophone Company.
It is likely that the SX-9 receivers were built by Howard Radio Co. or
by Wells-Gardner Co. - they were the major contractors in the Chicago
area, but RCA also would build radios as a contractor for some
companies. The SX-9 is a single-preselection receiver that uses all
metal octal tubes. It also had bandspread, built-in speaker, and
variable injection BFO. The power supply was built-in. The hams bought
SX-9 receivers in droves, mainly because it sold for about $90 with the
crystal filter. It was popular and with an external preselector, like
one built by Peak or similar, one could get pretty good results. By
itself, the SX-9 has all of the image problems one would expect from a
single RF stage receiver. Certainly price was the major factor in the
SX-9's popularity. As one would expect, the SX-9 can't compare to
several of its contemporaries, including the Super-Pro. That's not to
say that an experienced operator couldn't get great results from the
SX-9, especially with the addition of a preselector. Most hams know that
experience at using the equipment, whatever it is, sometimes is more
advantageous than the results gotten with the best receiver available used by an inexperienced operator.

photo above:
1936 Hallicrafters SX-9

Hallicrafters SX-28 vs SP-200
Super-Pro

Since the SX-28 is such a popular receiver and many hams
and collectors are
familiar with its performance I am including a direct comparison of
its features and performance with those of the SP-200 Super-Pro. Both were available
in 1940.

At $159, the SX-28 was a bargain compared to the $275
that the SP-200 usually sold for from discount dealers. With a built-in
power supply, frequency coverage from .54 to 43 MC and a respectable 8
watts of P-P audio from a pair of 6V6 tubes, the SX-28 did offer a lot
for the low price. Additionally, the Lamb Noise Silencer circuit was the
best noise limiter circuit available. Also, a ham band calibrated bandspread
was included. About the only accessory needed was the speaker.
Performance of the SX-28 was at the limits of the designs of the day.
But, the SX-28 had its quirks. The Antenna Trim doesn't function on the
lower two bands, double pre-selection is only used on the upper four
bands and the receiver initially had a noticeable hum level in the
audio. Both the SX-28 and SP-200 have a long warm up time, resulting in
drift that lasts for hours. Certainly, as far as sensitivity and the
ability to successfully allow world wide communications, the SX-28 performs equally as well
as the SP-200. When the selling price is thrown in, then it becomes
obvious why most hams bought the SX-28 rather than spend an additional
$116 for the few advantages the SP-200 offered. What advantages? The
SP-200 has better quality construction, it is certainly more a more reliable
receiver built with better quality components, the infinitely adjustable selectivity
is a better system, the front-end double preselection on all bands with
a more accurate dial readout due to the better tuning unit construction
and, of course, the more powerful and superior quality audio. But, in the long run,
hams felt that it just didn't make up for
the SP-200 disadvantages of having less frequency coverage per single
receiver, dealing with a separate power supply and the fact that the
bandspread is not calibrated in direct frequency readout. But certainly
the most important factor was cost - $275 versus $159.

W8TOW - Steve picked up this great example of the SP-10 at
Dayton 2007. It is serial number 720 with the matching sequentially serialized
power supply. It is the table top configuration. This SP-10 was once
owned by W2KW. Steve has gone through the receiver and it is in
excellent operating condition. A characteristic of the SP-10
circuit is that it can easily overload on modern, powerful AM signals when the
receiver is operating in AVC with an efficient antenna system. Steve's SP-10 exhibited this
typical behavior. Steve changed the bias source for the RF
Gain pot, removing it from the AVC line to eliminate the overloading
issue. Steve says that now the SP-10
is a pleasure to use on all signals and, that in most cases,
it will actually "out hear the 75A-4."

SP-10X

W9JDT - Bob has owned this very nice example of
the Super-Pro for about two years. It has the optional crystal filter
installed and is properly designated as the SP-10X. This receiver's
serial number is 979 and the matching power supply is serial number 987.
Sometimes non-sequential serial numbers do show up. This particular pair
was probably originally purchased from one of the many radio discount-dealers
of the thirties rather
than from the factory which accounts for the "close" but not sequential
numbers. Bob's SP-10X is partially re-capped and he uses it with a
Heathkit DX-60 on the 80 meter AM net.

SP-100/MRM-5

SMØAOM- Karl-Arne owns two of these Swedish
Markradiomottagare MRM-5 receivers. These are Hammarlund SP-100
receivers built especially for Swedish government customers. The
frequency coverage is different from the standard Super-Pro, covering
200kc to 400kc and .54mc to 10.0mc. The MRM-5 shown is a table top
version with a serial number in the 16XX range indicating 1937 as the
year of manufacture. The photo shows the receiver before clean-up and
alignment. Karl-Arne uses the receiver in his "thirties station." He
also owns MRM-5 sn 1352, a rack mount version along with the Swedish
manuals dated October 1937.

SP-200X with Faux Woodgrain Panel and Table Cabinet

KG5V - Chuck sent me this old photo of an SP-200X he
owned from 1957 to 1965. It was purchased when Chuck was a novice,
KN2TPU, out of a New York City newspaper ad for the price of $75. The
seller was not a ham and it was obvious that the receiver was not out of
a ham shack but had probably been in the living room or den of a genuine
radio enthusiast. Of note is the faux woodgrained front panel. It was
previously thought that only the SP-150 console was fitted with a faux woodgrained panel but it is obvious from the photo that this receiver is
an SP-200X. Note the knobbed switches over the phone jack and the AVC
switch. Also the crystal filter is integrated into the panel as the
SP-200X. The special cabinet was also faux woodgrained to match the
panel. All of the knobs were brown color and the band switch skirt was
faux grained. Speculation is that this receiver was built by Hammarlund for a
high ranking employee
(engineer? manager?) or for some NYC executive that warranted the extra cost of
producing this "special" SP-200X. Over the years while the
receiver was in Chuck's ham shack, visitors would comment on the unusual
look of this SP-200X receiver. Unfortunately, while Chuck was away in
the service his parents sold his faux woodgrained SP-200X
(1965) via another newpaper ad and for the same price of $75. Does
anyone know where this receiver has ended up today?

SP-100S Special Diversity
Receiver built for CODECO

KE7SE - Jack and his father own, this SP-100S
(SN:4167B) - a special build from Hammarlund for the
company Codeco in conjunction with the Civil Aeronautics Administration
- CAA. The receiver has two controls that are indications of its
intended use. The knobbed-switch to the left of the Main Tuning knob is
marked "HF OSC" - "INT" - "EXT" with another
knobbed-switch to the right of the Band Spread knob marked "DIV OUTPUT"
- "ON" - "OFF." These controls are indicators this SP-100S was part of a Space Diversity
receiving system. Two or three receivers would be used, each with their
own antenna separated from the other antennas by great distances
(usually 1000 feet separation in commercial/military set-ups.) To
achieve the diversity effect (reduction of fading) all
system receivers have their AVC lines connected together and their Diode Load
lines connected together. The result is that whichever system receiver is
responding to the strongest signal at any given time, that receiver
controls the AVC line and also has the maximum voltage on the Diode Load
line. To keep the system stable, one common Local Oscillator can be used
for all receivers allowing equal drift and common tracking of the system
frequency. The DIV OUTPUT switch allows individual receiver balancing
for equal response to a test signal. Does anyone have information on a
late-thirties Hammarlund Diversity Receiving Set-up?

SP-200SX

N2QEI - Pete has found this very early SP-200SX
receiver with the matching power supply. The power supply has the
terminal strip for the field coil connection for an electro-dynamic
speaker. The SX version of the SP-200 was built specifically for the ham
market since the receiver covers 160M up to 10M, 1.25mc to 40mc. Pete is
beginning the electronic restoration of the great example of the early
SP-200SX.

SP-100X with Original 12" Jensen Hi-Fidelity Speaker

N6YW, William Yates - owns this SP-100X (sn 2746) with the original power
supply (sn 2785) and the rarely seen, original Jensen Hi-Fidelity 12"
speaker. The standard speaker included with the purchase was a 10"
speaker but an optional speaker was the 12" Jensen. The speaker has two
cables with the standard Hammarlund "spade-lugs on terminal board"
connectors, one for the audio output from the receiver and one for the
field coil, which is connected to the power supply.

Also interesting is the addition of a cathode-ray tuning
eye tube to compliment the "difficult to read" Carrier Level meter. This
appears to be a professional installation as the proper "eye lid" bezel
is used in the mounting of the eye.

SP-100LX

WA7YBS - July 22, 2016 - The
serial number on this "LX" is 2730 which is about in the middle of
production meaning it's probably from mid-1938. The receiver has indications that it was supplied to
the Signal Corps and there's an MFP date stamp (4APR1945) on the chassis
and coil box.
The dust cover SN matches the receiver SN. The power supply that came
with the receiver seems to be somewhat newer with a serial number of
5164 which probably originally was for an early SP-200. The band spread knob is certainly not original. I also
suspect that the white pointer type knobs might not be original although
all the knobs are the same. It might have been something that the Signal
Corps wanted. On the rear
apron of the chassis are two Jones' plug sockets, one for remote relay and
one for B+, Fil
outputs. Neither are original but the installation is neatly done. As
for tubes, the Mixer and LO have later type tubes that are installed
into elaborate adapter sockets to preserve the original sockets. Also,
6V6 tubes installed substituting the original 6F6 tubes.

Update Jan 21,
2017 - I have inspected this SP-100LX and found it is mostly
complete and original with the exception of the coil box. It seems that
the Antenna/RF Coil for the 100kc to 200kc band was removed and the
antenna input connected to the 2nd RF coil. I'll need to find a good LX
coil box to harvest a replacement coil. I'm sure that a BC-779 coil box
would work (a lot easier to find.) No other serious problems found. Original audio transformers
and the meter checked okay. Restoration on hold until 100kc-200kc coil parts found.

The SP-400, SP-600 and Conclusion

The Hammarlund Super-Pro of the thirties was just what
Hammarlund had intended for it to be - a "standard" by which all other
communications receivers would be measured. By the time WWII began, the
Super-Pro had matured into a first-rate, ruggedly-built receiver that
the military could use without reservation or modification. Immediately
after the war, Hammarlund began to offer the new SP-400X Super-Pro. The
new receiver redesigned the wartime SP-200 by changing the frequency
coverage to the more standard .54 to 30MC (or 1.2 to 40MC for the SX
version) and changing the audio tubes in the later versions to 6V6 types with audio output of 8 watts with dual
audio output Z of 8 or 500 ohms. Also the IF frequency was changed from
465KC to
the more standard 455KC. Other than those changes, the receiver was
still very familiar to former SP-200 owners. The styling changes to the
front panel using a very thin smooth finish paint and miniscule knobs (that are normally not found on surviving examples
today) have resulted in very few SP-400 front panels remaining in good
condition. Most are found with severe wear around the control
nomenclature. Unlike its immediate predecessor, the SP-400 is usually
found in the table top cabinet rather than in a rack mount
configuration.
The SP-400 was only around for a few years, 1946 to 1948. It was not
built in large quantities and is not seen too often. Hammarlund was
biding their time and designing a totally new Super-Pro that would again
set the "standard" for what a modern communications receiver would be -
the famous SP-600 series. That the SP-600 owes a lot to its
predecessors, especially in design approach, is obvious. The
SP-600 is a well-known receiver with a plethora of information available
on the web, including our own article "Rebuilding the Hammarlund
SP-600" - link below in Navigation Index.

Today, most hams would choose the SP-600 receiver for a
vintage ham station since it is the most modern version available. Most
collectors are interested in either the SP-10, SP-100 or SP-150 because
of their rarity. But, those receivers in between - the SP-200s, both
military and civilian, and the SP-400, are incredible performers and are
now finally being recognized as such. A desire to
build "the best" regardless of cost defines all of the Hammarlund Super-Pro
receivers. They are
examples of what one of the best manufacturers in the United States
could build during a time when this country produced absolutely the
finest radio equipment in the world.

photo right top: SP-400 owned by W2EMN

photo right bottom: SP-600JX-21, early version from 1953
(no product detector)

Information Wanted:

As with our other articles on the
National HRO, the
Hallicrafters SX-28
and DD-1 andthe RCA AR-88 Series, we are always updating this webpage to
provide the most accurate information available. We depend on hams,
collectors and others interested in preserving our radio manufacturing
history. We are always interested in any receiver or any information
that seems to contradict any of the information presented here. Accuracy
is our goal, so let us know what you have or what you know.
Send us your Super-Pro serial numbers, we will add them to the Super-Pro
Serial Number Log. Eventually, as more and more numbers are gathered, a
more accurate picture of the Super-Pro production will be possible.

We are particularly interested in the following:

1. Any matched set of Super-Pro and Power Supply
sequential serial numbers or any matched sets that don't have sequential
serial numbers - this
will help to clarify how some sets have sequential numbers and others
don't. If you know your receive-ps history it will help.

2.
If you have an operational
SP-10 - please e-mail your opinion of the receiver's performance.

3. Any SP-10 with
the 600 ohm Z audio using in-line resistors and a phone jack - confirms
that this version was built and what its frequency coverage was.

4. SP-100LX versions with non-standard LF coverage, standard covered
100kc to 400kc in two bands. Some ads and Riders' suggest that 150kc to
300kc was the LF tuning using only one range. Has anyone seen any LX
with non-standard LF coverage?

5. Any variations seen in the Super-Pro receivers that are factory
original - like odd tuning ranges, or non-standard parts that are
original installations. Please also include serial number.

6. Any SP-200
or BC Super-Pro with a serial number higher than 30000 - more accurate
estimate of the total production.

7. Any R-129/U that is operational - what is the IF? Manual says the IF
is 465kc but the receiver's lowest frequency band covers 300kc to 540kc
which implies a different IF or non-continuous coverage on that range.

1.
Hammarlund Owner's Manuals for SP-10 and
SP-200 - These provide information on design intent and expected
performance in addition to schematics, alignment and other information.

2. Rider's Perpetual Troubleshooter's Manuals - Various
volumes contain information on the Comet -Pro, SP-10, SP-100 and SP-200. Many times
this is the exact same information from the owner's manuals.

3. U.S. Army Signal Corps - TM-11-866 - This Army manual
contains a wealth of information on the SP-200 series, specifically the
BC-779, BC-794 and BC-1004 receivers. It also includes the various power
supplies, the R-129/U receiver and detailed circuit descriptions and
drawings. TM11-896
provided details on the 1948 Wickes' modified BC-794 receiver.

4. QST - Various issues from 1936 to 1940 - Contain original
ads that provide a time line for engineering and model changes, company
performance expectations, sometimes interesting users.

5. Communications Receivers - The Vacuum Tube Era: 1932-1981.
Raymond S. Moore - The best reference book on communications
receivers, provided Hammarlund history and general specifications on the
receivers.

6. The Hammarlund Historian - Website - Provided the Oscar
Hammarlund history. The website provides a lot of information on the
SP-600 series.

7. Popular Mechanics - December 1937 issue contains the
article on the SP-100SX - shown in its entirety above in the SP-100
section.

photo above: Ad for the Comet-Pro installed in a console
cabinet. From "Radio News and the Short-Wave", March 1934. Note that the
plug-in coils are mounted in a coil storage rack located under the lid
on the left side of the cabinet. This is a later version of the
Comet-Pro shown with AVC.

Comet and Comet "Pro" Features

Introduced in 1931, the first version of the Comet Pro was called the
Comet "All Wave" Receiver. This version used 24A, 27, 35 and 47 type tubes.
The power supply rectifier was a
type 80. The IF frequency was 465KC and there were two stages of IF
amplification. No RF amplifier was used so only two plug-in coils were
used for each tuning range. The plug-in coil set covered 250M to
16M in four pairs. A 240M to 550M AM BC band coil set was available
rather early in production. Initially, the main tuning (WL and OSC) may
have had the dials on the outside of the panel. Later they were fitted
behind the front panel and viewed through cutouts in the panel. The
Comet receivers were sold as a chassis or in a console cabinet. Later, a
table cabinet was available that was a
combination of wooden back and sides (painted black) with a metal lid
and face plate, though some receivers were still sold as console "entertainment"
radios. A change to the
audio output tube came in 1932 with a type 27 taking the place of the
type 47. Additionally, the field coil speaker connections were eliminated and
an earphone jack installed with output terminals for a loud speaker with
output transformer. When this version was used and where thunderous
volume was desired, a separate audio amplifier was provided. By 1932,
the Oscillator Coil wiring was slightly changed and individual coil shields were added
for both plug-in coils. Possibly at this time the Comet was
renamed the Comet Pro.

In 1933, the entire receiver was given a
complete upgrade and the new name, Comet-Pro, implied that the receiver was now
"professional-quality" in both performance and design. The tubes
were changed to type 57 and type 58 in the front end with electron
coupled oscillators used in the LO and BFO. The audio tube was changed
to a 2A5 with an audio output transformer added with a 4K ohm Z output
and a tapped output for earphone operation. The Wave Length coils were redesigned slightly for
the new antenna input connections that allowed a dipole feedline to be
used. The
cabinet was changed to an all-metal fabrication, though the wooden
version was still available on request, as was the console cabinet. A short time later, a
Lamb-style crystal filter became an available option, followed by a 10
Meter coil set and by the end of 1933,
Amplified AVC was added to the options. This required the addition of
another tube, a 2B7 duplex diode-pentode for the AVC functions.

From 1934 through 1935 not too many changes are incorporated into the
Comet-Pro since Hammarlund was primarily working on the Super-Pro
receiver. A "Stand by" switch function may have been added since some
late Comet-Pros appear to have an extra toggle switch installed on the
panel. Coil IDs were changed sometime between 1933 and 1934 with the
engraved wooden handles changed to now use a recessed paper ID tag
protected by a plastic cover. There were probably more late improvements
to the Comet-Pro and I'll add to this description if more information
surfaces.

Performance Expectations Using
the Comet Pro

The 1933 and later Comet Pro receivers tune using separate OSC and WL
condensers and a four section condenser in parallel with both the OSC
and WL condensers that is used as a bandspread tuner. The power is turned on with the lower left knob which also
is the Tone control. Since AVC was not optioned on my Comet Pro, the
Sensitivity control (lower right knob) has to be adjusted for each
station's particular strength because the audio gain is always at maximum and is not adjustable. BFO
is tuned on with the toggle switch and the frequency control is the
"swing arm" on top of the BFO coil can. The Crystal Filter has
an "on-off" switch and a Phasing control. The design of the coils places
the ham bands about in the center of each range, so setting the OSC and
WL at "50" is a good place to start. The actual "peak" for the
WL dial will somewhat depend on the antenna impedance but it should be fairly
close to the OSC setting. The knobs nearest the arc scales
control the OSC (left) and WL (right.) The bandspread (center larger knob) can then be tuned in search of signals.
The Bandspread dial is illuminated and is projected onto a frosted
window. All tuning scales are 0-100 and require "scale versus frequency"
graphs to determine exactly where the receiver is tuned. The graphs were
in the Hammarlund Comet Pro manuals. The Comet Pro has good sensitivity and
the bandspread allows for easy tuning on all amateur bands covered. The 80 and 40 meter
bands are especially well spread out with the 40M band covered by a little over 100% of the bandspread
and 80M uses
over 200%. Surprisingly, SSB signals demodulate quite well
since we are using the Sensitivity control to set the volume. This
provides the correct ratio of BFO injection to signal at the detector
for good CW and SSB copy. CW signals are great on a Comet Pro. AM
signals sound a bit different since there isn't any AVC. This results
in the operator running the receiver with minimum front end
amplification and maximum audio gain. This is great for noise reduction
and QRM but full-bodied AM isn't really possible except on the AM BC band where
signals are at a constant high level. High power ham AM signals also
sound great but sometimes QSB (fading) will make enjoyment difficult as
the Sensitivity control must be constantly adjusted for rapidly changing
conditions.

photo: The 1933 Hammarlund Comet Pro with Crystal Filter
option but without the AVC option.

Images are not a problem with the coil sets DD, CC or BB but the highest
frequency coil set AA (10MC to 20MC) is plagued with images. This isn't
unexpected in a receiver without a TRF amplifier ahead of the mixer
tuning above 10MC. With the AA set, it is best to use the graphs to set
the desired frequency since the images are about the same strength as
the tuned signal. The addition of an aftermarket pre-selector would all
but eliminate the image problem on the AA range (see last photo in this
section.) Drift is rapid for the first 10 minutes of
operation, then settles down to a very slow drift that is standard for
pre-war receivers. For a 1933 receiver,
the Comet Pro is a fine performer and it would certainly be possible to
use it for vintage ham communications today although earlier versions do
not have a "stand-by" function.

Comet Pro Selling Prices

The Comet Pro list price was $150 without tubes.
However, it was usually offered in several different configurations. Generally,
prices were as follows:

With all options the price was usually around $140 in
1934. However, Leeds did offer a complete Comet-Pro for the total price
of $117 FOB, in 1934. The 1934 competition's asking prices were as
follows:

photo left: The
1933 Comet Pro chassis. Antenna terminal is center of chassis. Speaker
terminal is near the AC power cord exit.

Rebuilding Capacitors in the Comet-Pro Receiver:The
Comet-Pro uses mostly metal box multi-capacitor packages along with two
bath tub types and three paper wax type caps. The bath tub caps are
black wax filled and have no metal bottoms so the wax is easy to "dig out"
to remove the original cap and install a replacement inside the metal
tub. The tub can be refilled for authenticity. The Aerovox paper wax caps
have to have the rolled end "unrolled" and then the internal cap just
slides out and a new cap can be installed and the end re-rolled.
Cardboard circular end-covers go over the leads so when the new
capacitor is installed the capacitor ends are not visible. A small
amount of wax can be
added for a more secure seal on the rolled end. The four multi-caps in
metal housings are more difficult to rebuild. The housings are removed
from the chassis and the wires disconnected from the circuit. Carefully
unbend the flare on the eyelets that hold the bottom fiber cover to the
can and remove it. Using a heat gun to get the can hot enough and using
the wires to hold on to, pull the internal cap assembly out of the can.
When hot, this pulls out easily. Make up a replacement cap of either
three .1uf caps (four leads - three caps with one common) or the single
.5uf with two leads. Use the original wire from the old caps and adhere
to Hammarlund's color code in building the replacements. These can now
be installed inside the cans and waxed in place. Reuse the eyelets to
hold the fiber bottom to the can and be sure that the correct wires exit
in the proper order out the holes. Crimp the eyelets and the cap is
ready to remount and rewire. Be sure to note that the multi-cap that
connects to the mixer tube is wired with the common connected to chassis
and the other two multi-caps connect with their commons to the
appropriate IF tube cathode. The multi-cap with the single .5uf connects
to the detector tube. The Comet-Pro doesn't use very many capacitors
when compared to the Super-Pro receivers and only the four multi-caps
mounted in the square metal housings are time consuming to rebuild.

Comet-Pro filter capacitors are "wet electrolytics."
Theoretically, they are self-healing and as long as they still have their
liquid inside, they are still usable. The trouble is that the liquid can
leak out, or it can dry out but most often the solids settle out and
remain at the bottom of the can and the liquid no longer has the same
dielectric properties. Though I have found a few wet electrolytics that
seem to work okay, I usually replace them for reliability's sake. I
mount new dry electrolytics inside the original can. This can be done
many different ways depending on the tools available. First mark an
index line on the can and then cut the bottom off about .25" up from the
bottom. I drill a counter-sunk hole in the bottom piece of the can to install
a 4-40 flat head screw that holds a solder lug inside the can. The
center conductor is then tapped to hold a 4-40 screw and lug. The new
electrolytic is installed between the two lugs. Use a piece of heavy
paper rolled into a tube that just fits inside the upper can piece and
the lower can piece. Epoxy the paper at the joint being sure to get the
epoxy on both the paper and the inside of the can at the joint. Join the
two pieces using the index mark as your guide for correct fit and hold
in place with masking tape until the epoxy cures. Remove the tape, cover
the joint with silver paint and then the rebuilt cap is ready to reinstall.

Henry Rogers - June 2008

photo above: The
Comet-Pro with the PEAK P-11 Pre-selector. The P-11 adds two
stages of RF amplification ahead of the mixer-oscillator of the
Comet-Pro which greatly reduces images and adds a little gain in
the sensitivity. The Comet-Pro becomes a splendid receiver for
its design period when used with a separate RF pre-selector.

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